Transactions of the Royal Society of New Zealand,

Transactions and Proceedings of the Royal Society of New Zealand, Volume 86, 1959, Unnumbered Page

Transactions of the Royal Society of New Zealand, Published by The Royal Society of New Zealand, Victoria University Of Wellington, P.O. Box 196, Wellington, New Zealand Editor: J. T. Salmon, D.Sc., F.R.S.N.Z., F.R.E.S. Associate Editor: C. A. Cotton, D.Sc., Hon. LL.D., A.O.S.M., F.G.S., F.R.S.N.Z. London Agent: High Commissioner for New Zealand, 415 Strand, London, W.C.2 Printed by Otago Daily Times and Witness Newspapers Co., Ltd., Dunedin, New Zealand

External Morphology (Plates 12–13.) General Account The body of Evechnius chloroticus is hemispherical, adult specimens having much the same outline and size as an average sized grapefruit. The surface of the animal on which the mouth is situated is known as the oral or adoral surface, and in life is always held next to the substratum. It is flattened and usually distinctly sunken. The centre of the mouth lies at the adoral pole which is separated from the aboral pole, at the other extremity of the body, by the principal axis, which also corresponds to the vertical diameter or height. The anus is situated at the aboral pole. The outline seen when the test is viewed from above is the ambitus, and its diameter corresponds to the horizontal diameter of the test. It is usually quite circular, but may be pentagonal with smoothly rounded corners (Plates 12–13). The horizontal diameter, or simply diameter, is considerably greater than the vertical diameter, usually almost twice as much, so that in a specimen of 83 mm horizontal diameter the vertical diameter was only 44 mm, i.e., 1:1 8. The profile is usually well rounded, but occasionally specimens may be either subconical or depressed aborally. The whole exterior of the test is covered by a dense carpet of spines and pedicellariae (Plate 12), except in the area immediately surrounding the mouth, which is membranous and quite devoid of spines, although some pedicellariae are present. This area is known as the peristome and from its centre the five pointed teeth of the masticatory apparatus project. During life the tube-feet may be seen in five broad zones on the test waving freely in the water often beyond the outer limit of the spines. In preserved specimens they are contracted and not so obvious. In denuded specimens, which are often to be found washed up on beaches after the animal has died, the peristome has usually disintegrated so that it is represented only by a wide decagonal hole on the adoral surface (Plate 13). From it ten double columns of plates, bearing the numerous tubercles which support

the spines in life, are seen passing up to the aboral or apical extremity. Five of these columns are seen to consist of a double series of poriferous areas lying on either side of a tuberculated interporiferous area. These are known as the ambulacral areas or simply ambulacra, and each lies in a radius of the animal. The columns alternating with them are interradial in position and bear numerous tubercles. They are called interambulacral areas. A small circle of plates called the calyx, or apical system, is present at the aboral extermity of the test (Plate 12). It surrounds the anus and contains within it a specially modified sieve plate or madreporite which is readily distinguished by its large size and porous surface. Using these plates as reference points, Lovén (1892) established the method of orientation, now known as Lovén's Law, which can be applied to all echinoids. If the animal is placed on its adoral surface in such a way that the madreporite and the interambulacrum with which it is continuous forms the right anterior interradius it is found that one ambulacrum is anterior and one interambulacrum posterior. The ambulacrum immediately to the right of the posterior interradius is then counted as lying in radius I and numbering of the radii continues in an anticlockwise direction so that the anterior ambulacrum comes to lie in radius III and the ambulacrum to the left of the posterior interambulacrum is in radius V. The three most anterior ambulacra constitute the trivium while the two posterior ones form the bwium. The interambulacra are numbered in the same way following after the ambulacra, so that the madreporite always lies in interradius 2 and the posterior interambulacrum in interradius 5. For clarity it is conventional to use Roman numerals for radu and Arabic numerals for the interradii. The orientation described here is based on Lang's (1896) interpretation of Lovén's Law. Chadwick (1900), however, describing the radu of Echinus, has counted the right-hand side radius of the trivium as I, so that the most anterior radius becomes II instead of III. Chadwick gives no full list of references, so it is impossible to tell what authority he was following. However, the English translation of Lang appeared in 1896, four years previous to Chadwick's own publication, and as some passages of his text are almost identical with the translation it would appear that he had consulted Lang and that his different interpretation of the orientation is due to misunderstanding or else merely a slip of the pen. Colour. The primary and larger secondary spines are dull green with whitish tips, while the club-shaped secondary spines are made conspicuous by their brilliant white tips. The external epithelium covering the animal is deeply pigmented so that the peristome appears quite red, as do also the extended tube-feet. Under adverse conditions this pigment is released, probably due to disintegration of the epidermis, and so is not obvious in preserved specimens. One specimen was found which was quite brown in appearance, particularly the primary spines. This was thought to be due to a thin layer of the pigmented epidermis carried further than usual up the side of each spine, for when the spines were removed from the animal the epidermis covering them soon disintegrated to reveal their normal green coloration. Properly cleaned denuded tests are green in colour, varying from dark green to quite pale shades. Around the peristome the ambulacra remain white. Brightly coloured specimens are very beautiful with their intricate pattern of tubercles and pores. Ambulacrum (Text-fig. 1, figs. 2–5.) The ambulacra are five double rows of plates in each radius, extending down the test from the ocular plates of the apical system to the edge of the peristome. Each ambulacrum consists of three distinct zones of almost equal width. There are two outer or perradial poriferous zones (PZ), where the ambulacral plates are perforated by the canals leading from the ampullae to the tube-feet, and an inner

or radial interporiferous zone (IPZ), which bears most of the spines of the ambulacral area. The general pattern of arrangement of the tubercles of these spines is discussed in the section dealing with the spines. The suture line (SL) separating the two columns of plates passes down the centre of each interporiferous zone. Mortensen observes that the poriferous zones are about “two-thirds as broad as the interporiferous zones in the ambital region”, but adorally he describes them as “somewhat restricted”. This is certainly true, but the interporiferous zone is also equivalently restricted so that the relationship still holds (Text-fig. I, Fig. 3). Aborally both zones taper to nothing (Text-fig. 1, Fig. 2). Each ambulacral plate, or major, is composed of three smaller plates, each bearing a pore-pair. Such composite plates are termed trigeminate or oligoporous. In the Family Echinometridae the plates are more usually polyporous, the genera Selenechinus and Evechinus being the only exceptions, apart from the genus Echinostrephus which has one species, Ech. molaris, oligoporous, while the other, Ech. aciculata, is polyporous. The fossil species Echinometra prisca also is usually oligoporous, but occasionally plates are found bearing four pore-pairs. Evechinus and Selenechinus are more advanced in this respect than Echinostrephus molaris, however, for neither genus has a primary tubercle occurring regularly on every ambulacral plate, but instead, as Mortensen (1943) describes it, on “every second-fourth plate”. The plates without the primary tubercle are smaller and much more compressed than those plates which bear them. A small part of the tubercle is frequently shared in Evechinus so that superficially the plates appear polyporous. This condition is described as pseudo-polyporous. On the other hand, the oligoporous condition, as found in all species of Echinus, is most usual in the Echinidae, although in both Sub-families Echininae and Parechininae the polyporous condition has arisen. (Mortensen, 1943.) To say that the tubercles are borne on every second-fourth plate is, I think, an over simplification. I have found that near the peristome, the tubercles in general occur on every second plate, but occasionally several may be found in line all bearing primary tubercles (Text-fig. 1, Fig. 3), thus emulating the simple condition of Echinostrephus molaris (Mortensen, 1943). Near the ambitus the tubercles frequently occur quite regularly on every third plate, though occasionally a second plate will bear a tubercle and sometimes even a first. Towards the apex the arrangement becomes more irregular and the number of plates between tubercles is frequently four or five (Text-fig. 1, Fig. 2). In general the frequency of the primary tubercles increases as the ambulacrum approaches the peristome. Mortensen (1943), commenting on the plates without tubercles, says that they are “often excluded from the midline of the area, or even divided so that only a small inner part remains in contact with the median suture”. This is the case in one specimen figured by him, which has one compressed plate near the ambitus just occluded from the median suture. I was not able to observe any such plates. In one or two cases where the suture between plates was difficult to see externally such appeared to be the case, but examination of the interior soon showed that the plate in question did actually attain the midline. The manner in which the plates are fitted together conforms in general to the typical echinoid pattern, as figured by Cuénot (1948). The two outer plates are primaries (Text-fig. 1, Fig. 4, PS, PL), extending from the outer suture to the midline, while the third plate is inserted between them and completely occluded from the median suture. Such a plate is known as a demiplate (DP). However, majors which do not support primary tubercles are a little different (LP) in that the small upper primary plate does not usually extend as far as the midline, and so become a second demiplate. The lower component of each trigeminate plate is described by Mortensen (1943) as usually occluded from the outer edge of the area,

Text-fig.. 1.—Ambulacrum and Apical System. Fig. 1—Apical system of specimen 94 mm diameter. Fig. 2— Ambulacrum at the apex of the test. Fig. 3—Ambulacrum at the peristome. Fig. 4—Interior view of ambulacral plates near the ambitus. Fig. 5—Polyporous ambulacral plates. (All measurements expressed in millimetres. Figs. 1–4 all drawn to the same scale.) Abbreviations: A, anus; AD, adoral edge; ADP, additional demi-plate; AP, apical pore; DP, demiplate; EM, extension of madreporic surface; FIS, first pore-pair of inner series; FM, first major; FMS, first pore-pair of median series; G, genital; GP, genital pore; IS, inner series; IPZ, interporiferous zone; LP, low, compressed plate; MS, median series; OC, ocular plate; OP, ocular pore; OS, outer series; PB, periproctal membrane; PL, large primary; PM, peristomial margin; PR, partially resorbed pore-pair; PS, small primary; PT, periproctal plate; PZ, poriferous zone; SL, median suture line; SPT, small primary tubercle; STR, secondary tubercle of radial series; STZ, secondary tubercle of interporiferous zone; TBP, tubercle for pediccllaria; TP, plate bearing primary tubercle.

but while this may sometimes be the case it is usually seen that although the plate appears occluded on the outer surface of the test, internally it clearly shares in the suture line. The more regular arrangement of the pore-pairs on the internal surface of the test helps in determining the boundaries of plates, especially near the peristome, where they are very obscure. It is seen (Text-fig. 1, Fig. 4) that the members of each pore-pair are here widely separated and at first I found it difficult to correlate them with their external openings. By passing a hair through the canals leading to the interior, however, it was possible to establish the basic pattern. The outermost members of each pore-pair in the two most apical plates of a major pass almost straight down through the test, while the inner members travel obliquely a distance of several millimetres (OS, MS). On the large primary plate, however, it is the inner member which passes directly down, the other canal having to travel obliquely to almost meet the outer member from the central demiplate (IS). From the interior of the test, too, it can be noted how the openings of the pore-pairs from one plate abut against the neighbouring, less apical plate, and may even seem to be included in it. This is particularly true of the pores on the largest primary, almost all of which are contributed by the small primary, or outer demiplate, of the next major. The are formed by the pore-pairs on each plate is almost horizontal, so that the pores form three vertical series. This is distinct from the condition in Selenchinus where the pore arcs are almost erect. In species of Echinus it is a variable character. Ech. atlanticus has very erect pore arcs which form almost a straight line, but in most forms they are distinctly oblique. They may, however, become so oblique as to be almost horizontal so that the pore-pairs here also come to form three vertical series. This is the case in large specimens of Echinus esculentus (Mortensen, 1943). The inner and outer columns of pore-pairs are quite regular, while the median series is much more uneven. The sequence of events during growth may be seen in an average-sized specimen by tracing the arrangement of the pores from the apex down towards the peristome (Text-fig. 1, Fig. 2). The first three or four pore-pans, situated on the outer edge of each plate, are in a single vertical series and belong to separate plates which have not yet become compacted into majors. In the following seven or eight majors the lowest plate—i.e, the large primary—has tended to become excluded from the outer suture, but instead has extended considerably to-wards the midline. This has caused its pore-pair to move in towards the radius and thus constitute a regular inner series (IS). The pore-pair of the upper plate in each of these majors, however, remains quite close to the outer suture so that it helps to contribute to the outer series (OS). The median series (MS) first becomes evident in most specimens when the ambulacrum has progressed at least 10 mm or even further down the test. The upper primary or demiplate also tends to become excluded from the outer suture so that its pore-pair moves in a little towards the midline to constitute the median series, divergent from the outer column of pore-pairs, which is then composed only of pore-pairs from the central demiplate of each major. This arrangement holds for the remainder of the test, with the median series eventually becoming quite regular towards the peristome. In a very small specimen of 18 mm diameter it is difficult to distinguish a median series at all, for the pore-pairs all remain in contact with one another and are not at all separated as in older forms. Just below the ambitus, however, the poriferous zone widens and from this region to the peristome a median series can just be made out. It would seem, then, that growth of plates is not merely by deposition from the surrounding membranes, but must be by the addition of more calcite between the meshes constituting the whole plate, thus forcing the pore-pairs to move apart. In this case the plate must remain “alive” at least for some considerable time and grow continually by action of the spiculoblast living within it.

Although the majors are characteristically trigeminate, occasionally true polyporous plates are found, bearing four or even five pore-pairs. This recalls the condition found in the fossil species Echinometra prisca, where, however, the polyporous plates may have a higher incidence (Mortensen, 1943). Such a quadrigeminate plate is shown in Text-fig. 1, Fig. 5. In this major it is the upper primary plate (PL) instead of the lower (PS) which supports the main part of the primary tubercle, but otherwise the plate retains its usual form except that a fourth plate has been added as a small demiplate (ADP), almost occluded from the outer suture, to the apical edge of the major. However, another polyporous plate which bore five pore-pairs had quite an abnormal arrangement. It consisted of three primaries, all with pore-pairs belonging to the outer series. One demiplate was sandwiched between the two most aboral primaries and another large one lay along the adoral edge of the plate. Both were, however, occluded from the outer suture and remained instead in broad contact with the midline. The pore-pair of the most apical demiplate belonged to the middle series, while that of the lower one contributed to the outer series. The plate did not bear a primary tubercle. The presence occasionally of polyporous plates helps to emphasize Mortensen's (1943) contention that the “oligoporous or polyporous condition of the ambulacra is not of primary importance for classification”, but has arisen instead many times in separate evolutionary stocks. The number of ambulacral plates must obviously increase considerably as the animal ages. In a specimen of 18 mm diameter there were 27 ambulacral plates, while in a large specimen of 112 mm diameter there were 103—i. e., while the diameter has increased six times, the number of ambulacral plates has tripled. Using Mortensen's (1943) figures for the extremely small specimen of 6 mm diameter which he records, it is found that the diameter increases 18 times while the ambulacial plates increase to over six times the original number. Interambulacram (Text-fig. 2, figs. 5–8.) The plates of the interambulacra are present in five double rows alternating with the rows of ambulacral plates and thus occupying an interradial position on the test. A fully formed interambulacral plate is roughly three times as large as an ambulacral plate and very much simpler. Each is a single plate, undivided by the suture lines which traverse the composite plates of the ambulacra. The interambulacral plates are the important spine-bearing plates of the animal and in addition carry numerous pedicellariae. The arrangement of the tubercles is described in the section dealing with spines, but a brief summary will be given here of their distribution in a single column of interambulacral plates. Admedially, there is a row of secondary tubercles which is, however, not always present, especially in specimens of small diameter. It is not present in the interambulacral plates figured here (Text-fig. 2, Fig. 7), which were taken from a specimen of 60 mm diameter. Very close to these, moving out towards the radius, is a vertical series of small primary tubercles (SPT) followed by a row of secondary tubercles, each of which is developed towards the aboral edge of the plate (STP). A large primary tubercle (LPT) is present in the centre of each plate and is particularly conspicuous on the aboral part of the test. Finally, on the outer edge of each plate there are one or two secondary tubercles of the adradial series (STA). Where two such tubercles are present one is usually situated right next to the suture line between the ambulacrum and interambulacrum, with the other a little further in and rather more aboral. Between these series of tubercles numerous small secondary tubercles (MT), together with mihary tubercles for the pedicellariae (TBP), are scattered on every available space.

Text-fig. 2.—Interambulacrum and Peristome. Fig. 1—Peristome. Fig. 2—Buccal ambulacral plate. Fig. 3—Peristomial plate. Fig. 4—Fenestrated peristomial plate. Fig. 5—Interambulacrum at the apex of the test. Fig. 6—Fourth plate from the apex on the left hand side of the same interambulacrum. Fig. 7—Interambulacral plates from the ambital region. Fig. 8—Interambulacrum at the peristome. (All measurements expressed in millimetres. Figs. 7–8 drawn to the same scale.) Abbreviations: BF, buccal tube-foot; BP, buccal ambulacral plate; G, gills: GC, gill cleft; G4, genital plate No 4; IR, interradius; L, lip; LPT, large primary tubercle; M, mouth; MT, miliary tubercle; NP, newly formed interambulacral plate; OP, ophicephalous pedicellaria; PF, pore-pair; PL, peristomial plate; PM, peristomial margin; PMB, peristomial inembrane; R, radius; SPT, small primary tubercle; STA, secondary tubercle of adradial series; STP, secondary tubercle of series between primary tubercles; T, tooth; TBP, tubeicles for pedicellariae; TF, trifoliate pedicellaria.

New interambulacral plates, like those of the ambulacra, are added immediately behind the apical system, in this case behind the genitals, in the angle between each genital plate and the ocular plates on either side of it. Each new interambulacral ossicle first appears as a very small trianguler plate only just visible to the naked eye (Text-fig. 2, Fig. 5, NP). A small tubercle, miliary in size, soon develops in the centre of the plate and grows rapidly to occupy nearly its whole area. It is asymmetrical in shape, corresponding to the triangular shape of the plate (IL). By the time a further new ossicle and its tubercle have appeared on the other side of the genital, two or three pedicellariae have been added near the adoral edge (IR). The plate begins to square up and, although the primary tubercle still takes up most of the area of the plate, further small tubercles and pedicellariae are added, this time on its aboral side. From now on there is very little addition to the adoral side (2L). Growth continues, especially aborally, so that the plate becomes almost rectangular with its longest axis parallel to the median suture line (2R). This is accompanied by rapid growth of the large primary tubercle (LPT) while a small primary tubercle (SPT) appears in the centre midline. Expansion of the plate adradially begins so that the large primary tubercle becomes sur rounded on either side and aborally by a ring of tubercles and pedicellariae. Adorally, the ring is completed by those present on the most apical part of the next plate (Text-fig. 2, Fig. 6). Adradial and admedial expansion continues rapidly so that the fifth or sixth plate down becomes elongate parallel with the ambitus and therefore at right-angles to the previous direction of elongation. Expansion of the primary tubercle evidently slows down so that at the ambitus the tubercle finally comes to take up rather less than a quarter of the whole plate. It does expand slightly, however, so that its areole comes to extend over the suture line on to the next most adoral plate, thus helping to hold the plates firmly together (Text-fig. 2, Fig. 7). The areole is also much more sharply delimited than it is in the more aboral plates, with a distinct ridge becoming developed around it. Evidently when Mortensen (1943) wrote, “In general the areoles round the larger tubercles are narrow and indistinct”, he was referring to the most aboral ones. Past the ambitus, as the plates become more compacted, they become smaller once more so that the large primary tubercles take up correspondingly more of each plate, until right at the edge of the peristome they once again constitute almost the complete plate (Text-fig. 2. Fig. 8). Their areoles become contiguous, and secondary and miliary tubercles are reduced in number. The outer series of secondary tubercles persists, however, and becomes very regular. At the peristomial margin resorption is constantly causing plates to disappear The plates are made irregular by the development of deep gill clefts (GC) extending up as far as the third plate from the margin. In this way the two most adoral plates on either side become completely cut in half, and sometimes, due to resorption, the admedial portion with its primary tubercle will have disappeared while the adradial half still lingers on at the edge of the gill cleft (Text-fig. 2, Fig. 8, IR). The parts of the third and fourth plates immediately underlying the gills bear neither pedicellariae nor spines, and so are quite smooth in denuded specimens (PG). Formation and growth of interambulacral plates in young specimens or “imagos” of Echinus miliaris have been described by Gordon (1926) and appear to be effected in very much the same way as I have described for the addition of new plates in older specimens of Evechinus chloroticus. In a small specimen (18 mm diameter) the sequence of plates was the same as in older specimens. The fifth plate down from the apex corresponded to the fifth plate of a larger specimen in being the first plate elongate parallel with the ambitus, but as there were only 15 interambulacral plates in all the fifth plate in this case was situated almost at the ambitus. The interambulacra are described by Mortensen (1943) as becoming very constricted at the peristome and so narrower than the ambulacral areas. This is true,

for the plates above the gill clefts, but those surrounding them are actually caused to broaden out so that at the edge of the peristome the interambulacral areas are of the same size as or even slightly larger than the ambulacral areas. Yonge (1949) has described a method for aging plates of Echinus esculentus and claims that “careful horizontal grinding of these reveals rings of growth”. The same technique was tried with interambulacral plates of Evechinus chloroticus but no distinct growth lines could be distinguished. The gut of the animal is deeply pigmented and specimens are often found with the inner wall of the test quite pink, but evidently the pigment cannot be laid down permanently. It was suggested by Dr. Fell (personal communication) that possibly the rings formed by the trabeculae in the spines might correspond to growth rings, for seven to eight such rings can be made out in a spine of Echinus esculentus, which is known to have a life span of eight or more years. However, there can be no such correlation in Evechinus chloroticus as the spines of very small specimens had the same number of rings as those of large specimens. On the interior of the test, however, at both admedial and adradial ends of each plate, a number of ridges can be seen which are obviously associated with growth. They are rather inegular, and as again specimens of quite different sixes may have the same number of ridges they are evidently not seasonal or annual phenomena. The interambulacral plates of Evechinus appear to be of much the same pattern as those of Echinus, except that in the latter the plates immediately below the ambitus bear numbers of secondary tubercles, which have become so large that they are difficult to distinguish from the primary tubercles (Chadwick, 1900). Such plates appear to be carrying from 8 to 12 primary tubercles. The interambulacra of those echinometrids which are notable for their particularly long spines have become modified in a similar way, while those forms which live in very exposed situations have developed adoral petaloid ambulacra causing the interambulacral areas to be much more restricted than is the case in Evechinus chloroticus (Mortensen, 1943). Peristome (Text-fig. 2, figs. 1–4.) The peristome is that area of the animal which extends from the adoral edge of the calcareous plates of the corona to the mouth. Some authors (Lang, 1896) describe the corona and the peristome as together constituting the perisome. It is a tough fibrous membrane in which numerous calcareous plates are imbedded. At the oral edge it is terminated in a thick, circular lip (L) surrounding the projecting tips of the five teeth. In each radius immediately below the lips are the five pairs of buccal tube-feet (BF). These will be described in the section dealing with the ambulacial system. Each one is situated on a small buccal ambulacral plate (BP) roughly triangular in shape (15 × 25 mm) which is perforated by the excurrent and incurrent canals of the tube-feet. Around the edges of the plates numerous small tubercles (TBP) are scattered for the articulation of pedicellariae, which are mainly of the ophicephalous type (OP) although a few trifoliate (TF) may be present among them. The remainder of the peristome is relatively bare. Small calcareous plates (Text-fig. 2, Fig. 3) are scattered here and there over it, particularly in the areas corresponding to the ambulacra of the corona. Each plate bears several tubercles for articulation with the stalks of trifoliate pedicellanae, which are very abundant in this area, although their smallness of size and delicacy of structure do not render them very obvious. No pedicellariae of any other kind are present. In addition, the peristome contains a large number of small fenestrated plates (Text-fig. 2, Fig. 4) imbedded within it. Spines, such as are present on the peristome of Echinus, are completely absent in Evechinus.

The peristome forms the floor of the lantern coelom and as such, in each interradius, is continued at its adoral edge, as a pair of branching outgrowths, or gills (Text-fig. 2, Fig. 1, G), described later in the section dealing with Aristotle's lantern and the lantern coelom. In repose the peristome lies quite flat, but when the lantern is protracted, the peristomial area becomes pushed out to assume a blunt conical shape. In these movements of the lantern, it is seen how necessary it is to have the area of the body immediately surrounding it completely flexible. If the mouth were surrounded by the hard, immovable plates covering the rest of the animal the lantern would be inoperable. The diameter of the peristome increases with the size of the animal. In average size specimens it is usually between 20 to 25 mm in diameter, which is considered to be quite wide by Mortensen (1943), in comparison with other echinoids However, the peristome, expressed as a percentage of the horizontal diameter, decreases with age—e.g., in a specimen of 18 mm diameter it comprised 44% of the horizontal diameter, while in one of 98 mm diameter it comprised only 26%. Some members of the Family Echinometridae—e.g., Zenocentrotus kellersi and species of Heterocentrotus—carry spines on the peristome in the same way as do species of Echinus (Mortensen, 1943). These are not present, however, in either Evechinus chloroticus or Heliocidarts erythrogramma The peristome of H. erythrogramma is very similar to that of Evechinus except that tridactyl pedrcellariae are also occasionally present scattered among the trifoliate. There are also fewer calcarcous plates. In width, comparative bareness of the buccal membrane, and the presence of delicate, fenestrated plates imbedded within it the peristome of Evechinus conforms to the general pattern exhibited by echinometrids. Apical System (Text-fig. 1, Fig. 1.) The aboral extremity of the test in all regular echinoids terminates in a number of plates which together are known as the apical system. This may consist of two things of plates, an outer ring of oculars, or radials and an inner ring of genitals, or basals, surrounding the circular periproct. Evechinus chloroticus, however, shows an intermediate stage between this dicyclic condition, which is typical of most of the Echinidae, including Echinus, and the monocyclic condition where the oculars have all been drawn into the inner ring and are included between the genitals in a single ring surrounding the periproct. In Evechinus oculars II, III and IV all remain outside—i.e., they are exsert, while oculars I and V have become insert. The plates are sometimes given the name of “calyx” but MacBride (1906) objects to this term on the grounds that it has no homology with the structure of the same name found in crinoids. “Some zoologists have separated the ocular and genital plates under the name of ‘calyx’ from the rest of the corona, under a mistaken idea that they are homologous with the plates of the body or calyx of a Crinoid.” The ocular plates (OC) are five small ossicles in each radius which bear the terminal tentacles of the ambulacral canals. The term “ocular” refers to the fact that these tentacles were once thought to be light-sensitive and to function as cyes. Each plate usually bears one large spine on the surface nearest to the periproct, with several other smaller ones, and also pedicellariae, scattered here and there. The outer border is grooved to dovetail into the double row of ambulacral plates abutting against it. Each edge is produced to form a sharp corner, which Mortensen (1943) describes as “rather characteristic”. Two of the ocular plates, oculars V and I, are typically included in the limitation of the periproct—i.e., they are insert—which is the common condition in the Echinometridae, although a few members of the family have all oculars exert—e.g., Echinometra vindis, E. insularis, Zeno-

centrotus paradoxus and Heterocentrotus mammillatus (Morténśen, 1943). There is some variation in E. chloroticus, however. I have found ocular IV insert in one specimen, and several others in which it was almost insert. Mortensen (1943) comments on this tendency for further ossicles to be drawn into the inner circle. “Jackson has found one specimen with Oc. I, V and IV insert, and I have found the same to be the case in one specimen of 34 mm diameter … in one case Jackson found Oc. IV instead of V insert.” The size of the plates varies slightly about the periproct of the same animal, but in a specimen of 77 mm diameter they varied from 2.5 mm to 3 mm in width, and 1.5 mm to 2 mm in length—i.e., 13% of the diameter of the apical system. The pore is of the order of 0.2 mm wide which Mortensen (1943) apparently regards as unusually small. However, in his figure of the apical system he has drawn no pores at all showing in the oculars, while in describing the preceding species, Selenchinus armatus, where the pores are of the same order of size as in Evechinus chloroticus, he describes them as being comparatively large; so there is evidently some mistake. Possibly the specimen of Evechinus used for the figure was not properly cleaned so that the ocular pores were not showing clearly. They are usually rounded but may be slit-like in some specimens. The genital plates (G) lie in the five interradii and all abut against the periproct. They are larger than the oculars and like them vary in size. Generally those in interradu 4 and 5 are somewhat smaller than those in interradii 3 and 1, while that in interradius 2 is considerably enlarged to form the madreporite. In the specimen of 77 mm diameter, the genital in interradius 3 was 4 mm wide by 3.5 mm long—i.e., 31% of the diameter of the system. Each plate is roughly pentagonal in shape, the pointed end being directed away from the periproct. It bears the genital pore, which is quite large in both sexes (0.7 mm in the same specimen of 77 mm diameter) and so cannot be used for sex determination, as it is in some echinoids. In one specimen the pore of the genital was seen to have broken through the edge of the plate (Text-fig. 7, Fig. 5) after the same fashion as the pores in Zenocentrotus kellersi where it is the normal condition (Mortensen, 1943). There is usually one large spine at the inner edge of each plate, surrounded by several others of varying sizes. The tubercles of these, and particularly those of the large spines, have an asymmetrical boss and consequently areole, the edge lying next to the periproct having become quite flattened. The genital plate in interradius 2, as previously mentioned, is enlarged to twice the size of the other genitals and has developed a porous structure to function as the sieve plate or madreporite (M) of the ambulacral system. In several specimens the madreporic surface was continued on to one or both of the neighbouring genital plates. (Text-fig. 1, Fig. 1, MS). In these cases the madreporic ampulla must also be enlarged to extend under the accessory genital plates. This tendency to increase the madreporic surface apparently occurs quite frequently in other members of the Echinoidea, although Mortensen (1943) does not record it from Evechinus, nor mention it as commonly occurring in the Echinometridae. The only echinometrid he mentions is Anthocidaris crassispina, in which the madreporic pores in one specimen were observed to spread over genital 3. Lang (1896) remarks of this condition, “The madreporite, through which water flows into the stone canal, is not necessarily exclusively connected with the right anterior basal (genital) plate. On the contrary, the neighbouring genital plates, indeed all the five plates, and in isolated cases even the neighbouring interradial plates of the corona may be perforated by the afferent ducts of the stone canal. In Palaeechinus each basal plate is perforated by three pores, which are perhaps apertures of the stone canal, perhaps genital apertures, or else partly the one and partly the other. In no case, however, do the madreporic apertures extend to the radials (ocular plates)”. The oculars in Evechinus also, were never found to have the madreporic surface continued on to them. The madreporite may also bear several small spines and a few pedicellariae outside the

porous areas. There are usually three spines grouped together at the inner edge of the plate and one is usually present on either side of the genital pore. A few may also be found scattered here and there among the pores. In one specimen as many as 16 were counted. The periproct (PT) is circular to roughly elliptical in outline. In specimens from 60 to 98 mm in diameter it varied randomly from 5 mm to 7 mm in diameter and was never found wider than this even in the largest specimens. It consists of a tough membrane in which a number of plates are imbedded surrounding the slightly excentric anus. There are quite large spaces between the plates in older specimens, while in young specimens they fit closely together. There is an outer circle of large plates, within which lies a second circle of long, thin plates, sharply pointed proximally, which radiate out from the central anus. The suranal plate cannot usually be seen, although Mortensen (1943) claims to have seen it in “quite young specimens”. Small spines are present on the larger plates only, but pedicellariae may be present on the plates immediately surrounding the anus. More of the large outer plates are present on the periproct of very large specimens. Heliocidaris erythrogramma differs from Evechinus in this respect, that spines and pedicellariae are not so numerous on the plates of the periproct, and indeed on all the plates of the apical system. Measurements taken of the apical system of Evechinus show it to be rather small. Mortensen (1943), from a sample of 20, found it to be 12% to 15% of the horizontal diameter, and from a sample of 42 I have also found this to be the general size range in average size specimens (60 mm to 90 mm diameter). In smaller specimens, the apical system takes up a correspondingly larger percentage of the horizontal diameter. One of 18 mm diameter had an apical system of 4 mm diameter—i.e., 22%—while Mortensen found a very small specimen of 6 mm diameter to have an apical system of 2 mm diameter—i.e., 33%. Conversely, the larger the specimen, the smaller percentage of the diameter does the apical system take up— e.g., it was 11% in an animal of 97 mm diameter. Spines (Text-fig. 3; 4, Fig. 8.) The external surface of Evechinus chloroticus is covered with a thick carpet of green and white spines of varying sizes. Each spine consists of a slender shaft (SH), which in primary and the larger secondary spines is tapered to end in a blunt point, usually rather light in colour. Compared with other members of the Echinometridae, these spines of Evechinus, like those also of Selenchinus, Pachycentrotus and Caenocentrotus, are only of moderate size. In other echinometrids the spines are characteristically very long and well developed (Mortensen, 1943). The small secondary spines have brilliant white tips and are club-shaped, a very characteristic feature of the genus. The shaft has a ribbed appearance which is caused by the projecting ends of the calcareous wedges of which it is composed. These are revealed in cross section (Text-fig. 3, Figs. 4, 5). Neither the neck nor the collar regions are distinct in spines of Evechinus. Instead the shaft simply terminates in a conspicuous milled ring (MR), formed by the ribs expanding and curling over on themselves (Text-fig. 3, Figs. 1, 2 and 3). The base of each spine is smooth and rounded, ending proximally in a smooth socket, the acetabulum (AC), which fits over a corresponding tubercle (T) on the test. The tubercle is much larger in proportion to the acetabulum, and MacBride (1906) points out that this permits the spines of echmoids to have a very wide range of motion. It is supported on a prominent boss (B) from which it is delimited by a platform (PF), which in Evechinus, however, is distinct only on the larger tubercles. The spines are imperforate like those of all Echinometridae and also the Echinidae (Mortensen, 1943), so that there is no

Text-fig.. 3.—Spines and Spicules. Fig. 1—Large secondary spine. Fig. 2—Club-shaped secondary spine. Fig. 3—Base of primary spine. Fig. 4—T.S. primary spine. Fig. 5—T.S. club-shaped secondary spine. Fig. 6—L.S. secondary spine. Fig. 7—L.S. club-shaped secondary spine. Fig. 8—Spicule from a tube-foot. Fig. 9—Spicule from a buccal tube-foot. (All measurements expressed in millimetres.) Abbreviations: AC, acetabulum; AM, attachment marks of areolar muscles; BA, base; C, cortex; CH, club-shaped head; EP, end plate; MD, medulla; MR, milled ring; PF, perforation; PJ, projection; RZ, radial zone; SH, shaft; SP, septum; T, trabecula; TH, tapered head.

axial ligament present, with its accompanying perforations of tubercle and acetabulum. The milled ring and the area of the base immediately below it provide attachment for the radiole or areolar muscles (Text-fig. 4, Fig. 8, ARM). The rest of the base supports elastic connective tissue fibres (CT) which maintain the spine in place. The other end of the areolar muscle is attached to the areole (AR) of the tubercle, while the elastic fibres are attached mainly to the boss. Cuénot (1948) describes these fibres as “muscle interne” as distinct from the areolar muscle or “muscle externe”, but in longitudinal section it is seen to be exactly similar to the elastic connective tissue ensheathing the tube-feet. This interpretation would be more in keeping with its function which is merely to hold the spine in place on the test while the areolar muscles alone effect movements. Also seen in longitudinal section is the epithelium (EP) covering the areolar muscle and extending a short way up the spine. In very young echinoids the whole spine is covered by external epithelium, but this dies off as the spine grows and the cortical region is developed. About halfway between the milled ring and the test a nerve ring (NR) passes around the spine, enclosed within the epithelium. Transverse sections of spines (Text-fig. 3, Figs. 4 and 5) reveal a complex and beautiful pattern which is useful in systematic diagnoses. The centre of the spine consists of a sponge-like mass of calcite termed the medulla (MD), from which vertical septa (SP) of the same material radiate out. The septa are united together by rod-like trabeculae (T) passing between them. The septa and the trabecular wedges together constitute the radial zone (RZ), outside of which lies the thin cortex, in Evechinus made up of large end-plates (EP) continuous with the trabecular zones. It is these end plates of the cortex which are responsible for the ribbed appearance of the shaft. There is no sign of either cortical hairs or thorns. The frequency of the trabeculae is greater in some parts of the radial zone than in others. In the transverse section figured (Text-fig. 3, Fig. 4) there are three main concentrations of trabeculae. The calcite is darker in these regions so that they are quite conspicuous. They may possibly correspond to growth rings, but this is doubtful as three such rings were found in the primary spines of three specimens of differing sizes, 19 mm, 55 mm, and 88 mm diameter respectively. In the smallest specimen, however, the number of trabeculae in the two inner rings was much fewer than in the specimens of larger diameter. The septa were also much narrower. Transverse sections of the spines of Heliocidaris erythrogramma are very similar except that the rings of trabeculae are rather wider than in Evechinus. There is a marked colour difference, however; in those specimens which I was able to examine the radial zone was reddish brown, while the medulla was purple. Spines of adult Echinus have usually seven rings of trabeculae. As the age limit of this animal has been determined by other methods to be about seven years (Yonge, 1949), it is very tempting to regard the rings here as annual phenomena. The medulla is purple in colour with the inner part of the radial zone golden merging to green at the periphery. The cortical zone is much better developed than in Evechinus. Longitudinal sections ground of primary and large secondary spines (Text-fig. 3, Fig. 6) show the central medulla beginning as a constricted area halfway up the base, broadening suddenly just above the milled ring and then tapering gently up the shaft to the top of the spine. The green radial zone is sharply delimited a short distance above the acetabulum in the midline, but at the sides it is not evident until immediately above the milled ring. The remainder of the base is quite colourless. The small secondary spines present the same basic pattern except that the medulla, instead of tapering to the tip, broadens out so that the head comes to consist principally of medulla (Text-fig. 3, Fig. 7). This would account for the brilliant white tips of these spines. The relatively large proportion of medulla is also revealed in cross sections (Text fig. 3, Fig. 5), where the radial zone is only just evident;

with the end-plates of the cortex visible outside it. Comparison of both transverse and longitudinal sections of tapered and club-shaped spines shows that the former could easily have grown from the latter by the addition of proportionally more calcite in the radial zone, so that ultimately in the primary spine it becomes of greater width than the medulla. It can be seen from the figures that the medulla is of approximately the same diameter in both kinds of spine; so it evidently does not increase appreciably after the club-shaped stage. The distinction between the primary and secondary spines of Evechinus together with their corresponding tubercles appears to be rather subjective. The very largest spines, or primary spines, occur in a conspicuous, vertical row, down the centre of each column of interambulacral plates. (Text-fig. 2, Figs. 5–8, LPT). They vary in size according to their position on the test, being shorter aborally, increasing in length to almost double at the ambitus and then decreasing again as they approach the peristome. In a specimen of 98 mm diameter they varied 8 mm aboral, 16 mm ambitus, 11 mm adoral—i.e, 1:2:1.4. Mortensen (1943) describes the largest spines as rarely exceeding 15 mm in length, but most which I have seen are longer than this Specimens of 80 mm to 90 mm diameter usually had primary spines attaining about 20 mm at the ambitus. There is some individual variation, however. While a specimen of 98 mm diameter had primary spines 16 mm in length, one of only 50 mm diameter had them attaining 18 mm at the ambitus. Primary spines are also present as a double vertical series in the interporiferous zones of each ambulacrum, immediately surrounding the poriferous zones (Text-fig. 1, figs. 2–3, SPT). Near the peristome they are often borne on every plate, but on the remainder of the test vary usually between every second to fourth plate, and may become even more widely separated near the apex. They are smaller and less variable in size than those of the interambulacral plates. In an animal of 97 mm diameter they varied 6 mm aboral, 10 mm ambitus, 9 mm adoral—i.e., 1:1.6:1.5. A series of spines of similar size and vertical distribution is present in the interambulacra, admedial to the large primary ones (Text-fig. 2, figs. 5–8, SPT). Mortensen (1943) designates their tubercles as secondary. “In the interambulacra the larger secondary tubercles usually form a distinct vertical series admedially to each primary series…” As these tubercles are always well developed and quite as large as those which he describes as primary on the ambulacral plates, and the spines which they support are of the same order of size, I think they should also be described as primary. In that case there are two kinds of primary tubercle and spine, large (LPT) and small (SPT). The very large ones are only present in the interambulacra, while the small primary spines occur in both ambulacra and interambulacra, as described above. In addition to the three sets of primary spines, the test is covered by numerous smaller spines of varying shape and size, all of which are designated secondary spines. These may be tapered or club-shaped, and may form distinct vertical series on the test in much the same way as the primary spines, which they may even approach in size in large animals (Text-fig. 2, Fig. 7). The series, however, are not as constant as those formed by the primaries In average-size specimens there are three rows of such secondary spines in each interambulacrum and one in each ambulacrum. In the interambulacra (Text-fig. 2, Figs. 5–8) there are usually: (1) Admedially a single vertical row which in large specimens may be double, some becoming almost as large at the ambitus as the small primary spines. In some specimens, especially young ones, they are represented only by club-shaped spines. This series is lacking in the specimen figured here. (2) Adradially, there is an irregular row (STA) which in some specimens may become an alternating double series with each plate bearing two tubercles. In large animals they are all pointed, although the most aboral and adoral ones are conspicuously white-tipped, and in small specimens are club-shaped. (3) Between the large and small series of primaries a series of

small secondaries may be seen alternating with them (STP). These also may be club-shaped at the two extremities of the test. In very large specimens, usually over 100 mm diameter, further irregular rows may be present, in between the admedial secondary spines, and also between them and the small primaries. In the ambulacra (Text-fig. 1, Figs. 2 and 3) there is radially a single irregular median series (STR), though as Mortensen (1943) says, there may be two distinct vertical series in larger forms. In addition, the poriferous zones of large specimens may bear secondary spines in vertical rows between the three horizontal sets of pore-pairs (STZ). In small and average size specimens these are all club-shaped Over all the remainder of the test small club-shaped secondary spines are scattered either at random or in rings surrounding the primary spines. They vary in length from 2 mm to about 6 mm. The club-shaped nature of many of these secondary spines is described by Mortensen (1943) as “very characteristic”. In a specimen of 23 mm diameter only the interambulacral primaries and those of the ambulacra, except the most aboral and adoral, were pointed. All the rest were club-shaped. It seems that all the spines, including possibly even the largest primary ones, pass through a club-shaped stage during their period of growth. The three categories thus grade into one another extensively, but by correlating size, shape and position on the test it is possible to create some sort of classification useful for descriptive purposes. The function of the spines is two-fold. They are important organs of defence against predators in which they are apparently very successful, for I have observed, as did Mortensen (1943), that “regenerated spines are only rarely seen, a consequence, probably, of the strength of the spines—but possibly also due to the absence of attacking animals”. When the animal is disturbed the aboral spines immediately surrounding the anus, which normally stand up vertically about it like a palisade, close down to cover it. The spines surrounding the peristome, the other vulnerable region of the body, also close down over it. The spines help in the locomotion of the animal. They become the only organs of locomotion when the animal is moving over a loose, sandy surface. Over hard surfaces the tube-feet are often described as almost completely taking over this function. However, in an aquarium tank with a pebbly bottom, such as that on which the animal is frequently found, the spines were seen to assist the tube-feet considerably. The tube-feet, anterior with respect to the direction in which the animal was moving, were extended to take hold on fresh surfaces, while the “posterior” spines helped to propel the body, rather like oars, after them. As Mac-Bride (1906) suggests for Echinus esculentus, the spines are also used to steady the animal and prevent it from overturning under the unbalanced pull of the tube-feet. Pedicellariae (Text-fig. 4, figs. 1–4, V.) Dissection. Trifoliate and ophicephalous pedicellariae are easily obtained from the peristome by scraping. Tridactyl pedicellariae are not so plentiful, but can be readily seen even with the naked eye, scattered here and there on the test, especially on the adoral surface. Gemmiform pedicellariae occur in very large numbers on the aboral surface of the animal. All four types of pedicellariae should be examined under a microscope to reveal the details of their structure. Pedicellariae are found scattered over the whole test of Evechinus chloroticus. The four main kinds of pedicellariae typical of echinoids are all present. They are delicate, remarkably complex structures apparently derived from modified spines (Lang, 1896). Each consists of three parts: (1) The head (H), comprised of three jaws, or valves (V). In each of these a basal portion, or apophysis (A), with deep sockets (ADS) for the attachment of adductor muscles, and a distal blade (B), in

Text-fig. 4.—Pedicellariae, Sphaeridia and Spines. Fig. 1—Gemmiform pedicellaria. Fig. 2—Gemmiform pedicellaria, cleared and with mucous glands removed. Fig. 3—T. S. head of gemmiform pedicellaria at the level WX. Fig. 4—T. S. head of gemmiform pedicellaria at the level YZ. Figs. 5.–6—Sphaeridia removed from the test. Fig. 7—L. S. Sphaeridium. Fig. 8—L.S. miliary spine at its articulation with the test. (All measurements expressed in millimetres. Figs. 3–4 drawn to the same scale.) Abbreviations: AC, acetabulum; ADM, adductor muscle, AR, areole; ARM, areolar muscle; B, boss; BA, base; CSR, calcareous supporting rod; CT, connective tissue; EC, cushion of epithelium; EP, external epithelium; EPG, secretory epithelium of poison gland; H, head; ME, muscle envelope, MG, mucous gland; M, muscle layer; MR, milled ring; MT, mamelon of test; N, neck; NR, nerve ring; P, poison; PF, platform; PG, poison gland; PV, position of valve, S, stalk; T, tubercle; TC, tactile cushion; V, valve,

most cases toothed, may be distinguished. The adductor muscles (ADM) are large muscle bands passing between adjacent valves. The abductor muscles (ABM) are longitudinal fibres passing from ridges at the base of each valve to the calcareous supporting rod (CSR) of the stalk. The abductor muscles are seen to be a much less efficient mechanism than the adductors. (2) A flexible neck (N) consisting of longitudinal muscles and containing no calcareous ossicles. It is much reduced in the gemmiform pedicellariae. (3) A rigid stalk (S) consisting of what appears to be a firm calcareous rod but when sectioned is seen to be made up of calacareous strands disposed about an inner, non-calcareous core. I. Trifoliate pedicellariae (Triphyllous) (Text-fig. 5, figs. 1–3). These are small pedicellariae, about 2 mm long, found in large numbers on the peristomial membrane and scattered between the spines all over the surface of the test in both ambulacral and interambulacral areas. The head is very small (0.1–0.2 mm) consisting almost entirely of three broad, blunt jaws which are untoothed. From the distal part of the apophysis several fragile digitate processes (DP) project. These are characteristic of Evechinus and described by Mortensen (1943) as “remarkable”. It is hard to imagine what function they might have. Shallow sockets for the attachment of the adductor muscles of the valves lie immediately below these processes. They are separated by a broad median ridge (MRA) which is grooved to provide a surface of attachment for the abductor muscles. The whole valve is covered with a cushion of ectoderm. The neck is very long and flexible (0.9 mm) while the stalk is of almost equal length, containing a thin calcareous rod which swells out to provide the proximal surface of attachment for the abductor muscles. They are thought to function as cleaning organs, removing any small particles of mud and grit which may fall upon the animal. MacBride (1906), working on Echinus, describes the valves of this type of pedicellaria as being capable of independent action. Two blades work together, holding the object to be disposed of, while it is smashed by the third. The resulting fine powder may then be removed by the cilia of the epithelium. Such a function would, however, only be performed by those on the aboral surface of the test. The remaining trifoliate pedicellariae may deal with small animals or plants which chance to settle on the sea-urchin. In life they may be seen perpetually waving about from side to side. II. Ophicephalous pedicellariae (Text-fig. 5, figs. 4–5). These are rather larger than the preceding type (2–3 mm), but, apart from a larger head (0.6 mm), have much the same proportions of neck (0.7 mm) and stalk (1 mm). They are called ophicephalous because the head, at the end of the sinuous neck, looks rather snake-like. The valves are broadly spoon-shaped with crenulate edges, the crenulations being continued down on to the central ridge dividing the two adductor muscle sockets. Immediately below these sockets three ridges run across the valve, ending on either side in small processes (AP) which articulate with similar processes on the other two valves (Text-fig. 5, Fig. 5). The ridges are for the attachment of the longitudinal muscles of the neck. At the base of each valve is an irregular hoop or hook (AH), each of which articulates with its fellow, making dislocation of the valves almost impossible. The pedicellariae are scattered sparsely over the entire test, being especially abundant about the mouth, where they arise in large clumps from the buccal plates supporting the buccal tube-feet (Text-fig. 2, Fig. 1). They are thought to assist in feeding, as their great abundance about the mouth would suggest. MacBride (1906) says of these in Echinus that “with their powerful bulldog grip (they) assist in holding small animals such as Crustacea till the tube-feet can reach them and convey them to the mouth”, and they probably perform the same function in Evechinus. They have no very characteristic features and so are disregarded by Mortensen (1943) as having little taxonomic significance. III. Tridactyl pedicellariae (Tridentate) (Text-fig. 5, figs. 6–8). As stated by Mortensen (1943), there are two kinds of this type of pedicellaria. (1) Small

Text-fig. 5.—Pedicellariae. Fig. 1—Trifoliate pedicellaria. Fig. 2—Inside view of valve of trifoliate pedicellaria. Fig. 3—Lateral view of valve of trifoliate pedicellaria. Fig. 4—Ophicephalous pedicellaria. Fig. 5—Inside view of valve of ophicephalous pedicellaria. Fig. 6—Tridactyl pedicellaria. Fig. 7—Inside view of valve of tridactyl pedicellaria. Fig. 8—Small type of tridactyl pedicellaria. Fig. 9—Inside view of valve of gemmiform pedicellaria. (All measurements expressed in millimetres. Figs. 2–3 drawn to the same scale.) Abbreviations: A, apophysis, ABG, groove for abductor muscles; ABM, abductor muscles; ADM, adductor muscles; ADS, socket for adductor muscles; AH, articulating hook; AP, articulating process, B, blade; CSR, calcareous supporting rod; DP, digitate process; EP, epithelium; ET, end tooth; GP, groove for duct from poison gland; H, head; LT, lateral tooth; LW, lateral wing; MRA, median ridge of apophysis; MRB, median ridge of blade; N, neck; S, stalk; V, valve.

tridactyl pedicellariae (Text-fig. 5, Fig. 8) which to the naked eye are indistinguishable from trifoliate pedicellariae. The valves are 0.2 mm to 0.3 mm long and meet throughout, thus at first sight appearing rather like small ophicephalous pedicellaria, from which they are soon distinguished, however, by their lack of basal hooks. The neck is very long and flexible (0.5 mm) and the stalk, with its club-shaped calcareous rod, of almost equal length. They are found mainly with the trifoliate pedicellariae on the peristome. (2) Large tridactyl pedicellariae (Text-fig. 5, figs. 6–7), which are 4 mm to 5 mm long and so may be readily detected on the test, where they occur mainly on the adoral surface. The head, as Mortensen (1943) observed, is very large, usually about 2 mm long, while the neck and stalk are both of the order of 1 mm in length. The blade of the valve is usually twice as long as the base and rather slender, about 0.3 mm wide. The valve is, however, quite thick, especially near the base. On the outer surface a low median ridge (MRB) is present. The three blades meet each other distally and here their edges are coarsely dentate. On the inner surface a high median ridge (MRA) separates the two deep sockets which provide attachment for the adductor muscles. Grooves for the abductor muscles and lateral articulating processes (AP) are also present. Mortensen (1943) claims to have seen all transitions between the large and small forms. However, I have not been able to find any such transitional forms, although in the large type there are some variations in shape. The valves are usually long and slender, but some have been found that are quite broad and almost spoon-shaped. The tridactyl pedicellariae probably assist in feeding, passing small animals to the tube-feet and thence to the mouth. Also, as MacBride (1906) suggests, this type of pedicellaria may “seize and destroy the minute swimming larvae of various sessile parasitic animals, which would otherwise settle on the delicate exposed ectoderm of the sea-urchin”. IV. Gemmiform pedicellariae. (Globiferous) (Text-fig. 4, figs. 1–4). These are important taxonomically as they are used to differentiate between the families of the Sub-order Echinina (Mortensen, 1943). In Evechinus they are quite conspicuous because of their large size and great abundance on the aboral surface of the test. The external epithelium covering them imparts a dark red coloration which fades, however, on preservation. The valves are quite small (0.6 mm), consisting of a long narrow blade terminating in a sharply pointed inoculating end-tooth (ET) and a rather smaller, lateral unpaired one (LT). The possession of this lateral, unpaired tooth is the most important taxonomic character of the Family Echino-metridae. In Echinus the lateral teeth are more numerous and always paired (Mortensen, 1943). The sockets for the adductor muscles are very deep and divided by a high median ridge (MRA). They extend out on either side in the form of characteristic lateral wings (LW). Articulating processes (AP) and grooves for the attachment of the abductor muscles (ABG) are also present. Mortensen (1943) describes these muscles as being much longer in Evechinus than is usually the case in the sub-order Echinina. In this, he points out, Evechinus resembles Loxechinus albus, a specialized member of the Sub-family Parechininae, Family Echinidae, and is also superficially similar to the genus Strongylocentrotus, where the neck is, however, capable of much greater movement, and on sectioning is seen to be more specialized in the possession of circular muscle lying inside the longitudinal fibres. As a rule, in both Families Echinidae and Echinometridae, there is no neck, the head resting instead directly on the upper end of the stalk. This is the condition in Echinus (Mortensen, 1943). Associated with each valve is a pair of poison glands (PG) lying above the valve to almost obscure it, and extending down to the supporting rod so that the neck is also hidden. In this matter I find Mortensen's (1943) figure difficult to understand, for he has drawn the glands as shorter than the valves, thus revealing the base of the valves and the entire neck. This gives the head of the pedicellaria a globular shape, whereas. in all specimens I have observed, it is quite pear-shaped. That the gland is paired confirms the position of Evechinus

in the Echinometridae, for it is single in both the Echinidae and the Strongylo-centrotidae (Mortensen, 1943). A short common duct leads from the glands at the top of the head, near the end-tooth. Mortensen (1943) says, “the poison glands are not very large, but a thick investing skin makes the head of these pedicellariae rather thick and clumsy”. This observation is not based on sectioned material, but merely on external appearance. In sections which I have cut the epithelium (EP) is seen to be quite thick, but the glands are also large, occupying most of the volume of the head. Each gland is surrounded by a layer of longitudinal muscle (MM), internal to which is a cuboidal epithelium (EPG) for elaboration of the poison. Mortensen found it difficult to ascertain whether the mucous glands (MG) were present or not. I found these relatively easy to distinguish in preserved specimens, lying halfway up the head, between each pair of poison glands. They are a lighter colour than the poison glands and have a translucent appearance. In sections stained with Ehrlich's haematoxylin they take on a dark purple colour. A sensory, or tactile, cushion (TC) is present on the inner surface of each valve, just above the adductor muscles. Most authors agree that the gemmiform pedicellariae of echinoids are used for defence, particularly against starfish. This would agree with their distribution on Evechinus, for they are limited to the aboral half of the test. Apparently the stimulus is first chemotactic, which causes the valves to open wide. When the enemy draws close and the tactile cushions are touched this second stimulus causes the blades to close violently. The wounding of the enemy then renews and intensifies the chemical stimulus (MacBride, 1906). Cuénot (1948) points out that the pedicellariae never seize other parts of their own body such as tube-feet or spines, and never attack any of the commensal animals living on the test. Sphaeridia (Text-fig. 4, Figs. 5–7.) Dissection. The sphaeridia are very small and difficult to detect. They may be seen, however, by scraping carefully with a scalpel along the centre of an ambulacrum, at the same time examining the area with a hand lens under strong light. The small tubercles with which they articulate on the test may also be seen by examining a denuded test along the midline of an ambulacrum near the peristome with a hand lens. Sphaeridia were first discovered by Lovén and have since been found present in all members of the Echinoidea except the cidarids. They are quite numerous in the Echinometridae but lack any distinctive characters, as is indeed the case in all families of the Sub-order Echinina, and so are of no taxonomic importance (Mortensen, 1943). They are present in the midline of each ambulacrum, especially near the peristome. Each sphaeridium in Evechinus consists of a head and a stalk. The stalk (S) is of dense calcite bearing green pigment. In longitudinal sections (Text-fig. 4, Fig. 7) it is seen to have a lattice-like structure, very much like that of the plates of the test. At its proximal end it is slightly hollowed to articulate with a small tubercle on the test, while distally it is continuous with the head of the sphaeridium. Its sides provide attachment for the muscle envelope (ME) which surrounds the base of the sphaeridium and effects its movements. The head (H) of the sphaeridium is usually unpigmented and rather refringent. Sometimes it is smoothly rounded (Text-fig. 4, Fig. 5), but it may be extended by an irregular distal projection (Text-fig. 4, Fig. 6). Microscopically it is described by Lang (1896) as being concentrically laminated in all echinoids, and Chadwick (1900) figures this condition for Echinus. In Evechinus a lamination shows in transverse sections of the head as it lies along the surface of the test, but in strictly longitudinal

Text-fig. 6.—Internal Anatomy. Adoral View with Lantern Removed. Abbreviations: AR, aboral ring; ADG, adoral part of gonad; AO, axial organ, AP, ampulla; APG, apical coalescence of gonad; CL, collateral canal; CLB, branches to collateral canal; D. diverticulum; EMC, external; marginal canal; FLI, first downward loop of intestine; FLS, first loop of stomach; GD, gonoduct; IMC, internal marginal canal; INT, intestine; ML, mesenteric ligaments; OE, oesophagus; R, rectum; RAC, radial ambulacral canal; SI, siphon; ST, stomach; ULI, upward loop of intestine. tudinal sections (Text-fig. 4, Fig. 7) this is not evident. However, as Lang points out, it shows no “lattice-like perforated structure” like the rest of the skeleton and for that reason he thinks it may possibly correspond to the cortical layer of the spines. A circular ganglion (NR) passes round the base of the sphaeridium and the whole structure is completely covered by a ciliated epidermis (EP) continuous with that covering the exterior of the test. The complete sphaeridium is only of the order of 1 to 1.5 mm high. At its base the epidermis is raised in a small cushion (EC) and here the cilia are said to be especially long. I was not able to demonstrate this in Evechinus, but as I have found the detection of cilia difficult in most parts of the animal I would hesitate to say that they were not present. The sphaeridia are thought to be modified spines (Lang, 1896) which function as sensory organs, probably indicating to the animal its position with regard to gravity. As the animal shifts its position the heavy head of the sphaeridium would come to lie on different parts of the epidermal cushion, which is in turn closely associated with the nerve ring. In some echinoids—e.g., Clypeaster—the sphaeridia lie in completely closed cavities and thus come to resemble the statocysts of other phyla (Cuénot, 1948). However, such a “balancing” function has not yet been positively demonstrated for the sphaeridia of echinoids, although in experiments quoted by Cuénot (1948) it seemed that animals with their sphaeridia removed

took longer to return to the normal position than urchins with the normal complement of sphaeridia, which had been similarly displaced. Aristotle's Lantern (Text-fig. 7.) Dissection. The relationships of the ossicles comprising the lantern are easily studied on the lantern in situ. For more detailed study of individual ossicles a lantern deprived of its muscles, by cleaning in potassium hydroxide, is necessary. The mouth lies within the peristome (Text-fig. 7, Fig. 6, P) in the centre of the adoral face and is directed downwards. From it the pharynx (PH) leads vertically upwards, enclosed within Aristotle's lantern. This is a complex assemblage of ossicles and muscles used mainly for feeding, but with the subsidiary function of respiration. The lantern consists of 35 pieces, 30 of which are used for mastication. The largest and most prominent of these are the five sturdy pairs of jaws, each pair of which fuses in an interradius to form an alveolus (AL). These have the form of hollow, triangular pyramids (Text-fig. 7, Figs. 1–2), the inner cavities of which are traversed by the slender, more fragile teeth (T). The two lateral faces of the alveolus are marked with fine transverse, slightly sinuous grooves (Text-fig. 7, Fig. 2, GLW) for the attachment of the radical muscles connecting neighbouring alveoli. These grooves are continued beyond the inner edge of the alveolus, thus presenting a toothed appearance. The outer face of the alveolus (Text-fig. 7, Fig. 1) is smooth and slightly convex, the suture (ALS) between the two constituent jaws showing clearly in an exactly interradial position. This external wall is incomplete aborally, being perforated by a large triangular foramen, the foramen externum, or foramen magnum as it is sometimes called (FE). The aperture through which the tooth projects at the top of the alveolus is known as the foramen basale (FB). It is separated from the foramen externum by the arch formed by the fusion of a pair of ossicles, the epiphyses. The epiphyses (E) bear two small styloid processes (SP) which lie against the teeth, like the sides of a groove. A toothed epiphyseal crest (EC) runs distally from each process to the line of fusion of the epiphysis from each side (Text-fig. 7, Fig. 7). It appears to be purely ornamental. The arched outer surfaces of the epiphyses provide attachment for the protractor muscles of the lantern. The internal cavity of each alveolus is occupied by a tooth, which projects from it adorally as the externally visible tooth, and aborally as the soft, slightly coiled root (RT). This soft root is enclosed within a pocket-like projection of the transparent roof of the lantern coelom, known as a dental pocket. The root of the tooth is a region of continuous growth, which is described by Cuénot (1948) as being effected by the fusion along the midline of two lamellae to form a hollow cone. The compaction and hardening of the tooth is caused by successive hollow cones piling up on each other and fusing. The calcareous part of the tooth is curved so that its convex side fits into a groove on the concave inner surface of the alveolus. The concave side of the tooth bears a flattened keel, or carina. The rotulae (Text-fig. 7, Figs. 4, 6, 7, R) are five, more or less flat, rectangular ossicles lying radially between adjacent alveoli, looking rather like the spokes of a wheel. Slight grooves are present on the lateral faces of the alveoli where the rotulae articulate with them. Their main function appears to be support, both for the overlying compasses, and to help unite the alveoli. They also provide aboral attachment for the five radial pairs of ligaments which pass up the sides of the pharynx. Lying on top of the rotulae, and therefore also in a radial position, are five delicately shaped ossicles, known as compasses (Text-fig. 7, Figs. 6, 7, C). These have nothing to do with the feeding function of the lantern, but are concerned

Text-fig. 7.—Aristotle's Lantern. Fig. 1—Outer surface of alveolus. Fig. 2—Inner surface of alveolus. Fig. 3—1.S. alveolus. Fig. 4—Rotula. Fig. 5—Axial surface of auricula. Fig. 6—Lantern after removal of two alveoli to show pharynx. Fig. 7—Aristotle's lantern in situ. (All figures drawn to the same scale.) Abbreviations: AA, ambulacral apophysis; AL, alveolus; ALS, alveolar suture AO, axial organ; AP, ampulla; AR, ambulacral ring; AU, auricula; AX, axial end; C, compass; CA, carina; CDM, compass depressor muscle; CEM, compass elevator muscle; E, epiphysis; FB, foremen basale; FE, foramen externum; FR, facts for rotula; G, gills; GC, gill cleft; GLW, grooved lateral wall of alveolus; GRM, groove for retractor muscles; IA, interambulacral apophysis; IM, intermediate muscle; L, lips, LC, lantern coelom; LL, long ligament of pharynx; MES, mesentery; OE, oesophagus; OHC, oesophageal haemal canal; P, peristome; PH, pharynx; PM, protractor muscle; PV, Polian vesicle; R, rotula; RAC, radial ambulacral canal; RM, retractor muscle; RR, radial ridge of pharynx; RT, root of tooth; SC, stone canal; SL, short ligament of pharynx; SP, stylord process; T, tooth.

instead with respiration. They are slender, curved structures, with their distal extremities bifurcate, and slightly projecting over the edge of the lantern. About halfway along their length they are thickened and grooved to provide attachment for the compass elevator muscles. There appears to be some confusion in terminology in the literature where the rotulae and compasses are concerned. Lang (1896), for example, describes the rotulae as “sickles” or “falces”, while he reserves the term “rotulae” for what are now known as “compasses” with the term “radius” as an alternative. Chadwick (1900), on the other hand, while denoting the rotulae as such, describes the compasses as “radii”, which usage is now generally employed. Bather (1900) uses the term “brace” instead of rotula. The perignathic girdle (Text-fig. 7, Fig. 5) must also be described in conjunction with the lantern, for the two are physiologically closely connected, the girdle being present only in those sea urchins which also have a lantern. It consists of ambulacral and interambulacral plates from the margin of the peristome, which have been bent inwards to form processes known as apophyses to which the main muscles of the lantern are attached. The ambulacral apophyses (AA) are large ossicles about 1 cm high, arising from the peristomial margin on either side of the ambulacral suture. They incline together so that their free ends unite to form an arch over each radial canal of the ambulacral system and its associated vessels. There are thus five of these arches, or auriculae, in radial positions surrounding the lantern and providing attachment for the five pairs of large retractor muscles. The interambulacral apophyses (IA) are not so well developed. They project about 5 mm into the body cavity as a continuous ridge uniting neighbouring auriculae. Their inner or axial surface is grooved for the attachment of the protractor muscles of the lantern. They also bear gill clefts (GC). Three sets of muscles (Fig. 7) are involved in the feeding movements of the lantern. The most obvious are the five pairs of large protractor muscles (PM) running from the epiphysis on each side of the arch of the jaw to the interambulacral apophyses. These protrude the lantern through the peristome, at the same time tipping the alveoli back and bringing the teeth together. They therefore also function as jaw adductor muscles. Indeed in the earlier lierature (Lang, 1896; Chadwick, 1900) this is described as their only function. They are counteracted by a second set of large muscles, the retractor muscles (RM), which also act as abductors. They run horizontally from each side of the auriculae to well defined grooves on either side of the outer face of the alveolus. Their contraction withdraws the lantern into the body, and at the same time draws the teeth apart, opening the mouth. The intermediate, or radial muscles, are not so easily observed as they lie between neighbouring alveoli, attached to the sinuous grooves running along the lateral faces of each alveolus. Once the lantern has been protruded, it is made to move spirally to the left and to the right by their contraction. The two remaining sets of muscles operate the compass ossicles in respiratory movements. The pentagonal ring of muscles connecting neighbouring compasses as they lie on the base of the lantern, function as compass elevators (CEM). The raising of the compasses also raises the thin membranous roof of the lantern coelom, within which the lantern is enclosed. This reduces the pressure in the lantern coelom and causes the gills, which are feathery extensions of the peristome, to be withdrawn. Running distally from the bifurcate tips of each compass to the interambulacral region of the perignathic ring, are the fine compass depressor muscles (CDM). Their contraction lowers the compasses and thus the roof of the lantern coelom, increasing the pressure and causing the gills to be everted. The oxygen thus taken into the lantern coelom is able to diffuse through its extremely thin, transparent walls into the cavity of the main coelom. The dental pockets about the teeth must increase the surface area for gaseous exchange with the coelom in much the same

way as the gills do with sea water. Although this complex mechanism has been evolved for respiration, it is generally considered, however, that the major part of the respiratory function is carried out by the tube-feet. It may be seen from the foregoing description that Aristotle's lantern in Evechinus adheres to the general plan exhibited by the gnathostomous Echinoidea. In shape and disposition of both ossicles and muscles it closely resembles Echinus and like it possesses a camarodont type of dentition (Mortensen, 1943) by virtue of the epiphyses fused to form an epiphyseal arch, and by the carinated teeth. In Echinus and all echinids there are no supporting processes for the teeth such as the epiphyses bear in Evechinus, Heliocidaris erythrogramma and other echinometrids (Mortensen, 1943). The styloid processes of H. erythrogramma, however, are not so strongly developed as those of Evechinus. The auriculae of Echinus and Evechinus are apparently quite similar, being fairly broad, with a broad area of fusion between the apophyses. In average sized specimens of Evechinus this extends over approximately 4 mm, while in H. erythrogramma it is much narrower, being less than 1 mm in a specimen of 75 mm diameter. It would possibly be wider in larger specimens, which may attain 100 mm diameter. Alimentary System (Text-fig. 6; 7, Figs. 6;8;9; 10.) Dissection. The dissection to show the alimentary canal of Evechinus is made difficult by the large size of the gonads, which tend to obscure the other contents of the body cavity, and also by the extreme thinness of the gut wall Strong mesenteric ligaments attach the intestine to the gonads very firmly and must be previously cut if the intestine is lifted or moved in any way. If they are not cut, the intestine wall gives way and its mass of contents spills out to obscure the dissection. The great convolution of the alimentary canal, particularly the intestine, makes its course rather difficult to follow. For general laboratory dissection, where only one animal is available with which to demonstrate all systems, it is best to dissect from the adoral surface. By cutting the test around the peristome with old scissors or forceps, the lantern is completely freed and gently tipped so that the stone canal and axial organ may be located and freed. The mesentery holding the oesophagus in place must also be cut. The whole lantern can then be lifted out of the body cavity and placed to one side, while still retaining its connection with the oesophagus (Text-fig. 6). To demonstrate the alimentary canal alone, however, I have found the best method to be that of removing the aboral half of the test piece by piece with strong forceps, leaving until last that part immediately surrounding the apical system. The rectum, stone canal and gonoducts are then carefully severed immediately below the apex. The aboral halves of the gonads must be then removed to reveal the large intestine coiled around the test in undulating dorso-ventral folds. In the five radii the narrower dorso-ventral loops of the stomach may be observed. The oesophagus and rectum may be seen situated in radius III thus giving a reference point for correct orientation (Text-fig. 8). To observe the whole course of the stomach and its attendant siphon, the intestine and the rest of the gonads must be removed (Text-fig. 9). Spilling of the contents of the intestine is unavoidable at this stage but they may soon be removed by gentle washing. Aristotle's lantern must be opened to observe the pharynx. This is best done by removing, with forceps or old scissors, a whole alveolus, and then one jaw from either side of it, at the same time clearing away the accompanying muscles. The pharynx (PH) is that part of the digestive tube which is enclosed within the lantern (Text-Fig. 7, Fig. 6). It has a diameter of the order of 7 mm and a length corresponding to the height of the lantern—i.e., about 20 mm. Five radial ridges (RR) of connective tissue pass down its sides giving it a pentagonal shape.

About the mouth they become expanded to form five lobes, alternating with the interradial teeth. On both sides of each ridge are a pair of ligaments (LL) the adoral ends of which are fixed to the internal parts of the jaws. Aborally they are attached to the rotulae. Five other shorter pairs (SL) come from the inferior part of the pharynx to pass on to the jaws quite near the teeth. The pharynx leads into the narrow, much convoluted oesophagus (OE) (diameter ca. 5 mm), which continues upwards along the vertical axis of the animal, together with the axial organ (AO) and stone canal (SC), almost reaching the periproctal region. From there it bends on itself and redescends to the level of the lantern, to pass out horizontally in radius III between the rectum and the first loop of the intestine to meet the wider, thin-walled stomach (ST). Sheets of mesentery unite the oesophagus to the axial organ and to the diverticulum of the stomach. The mesentery is also attached to the test in the apical region, thus suspending the oesophagus with the axial organ and attendant stone canal in the central axis of the animal. In some specimens the oesophagus has a striated appearance due to the presence of rows of pigment passing along it. The stomach (ST) in fresh specimens is a yellowish-orange colour, and is extensively sacculated. It has been variously described as the “first or inferior spiral” (Lang, 1896; Bonnet, 1925), “direct canal” (Delage and Hérouard, 1903; Text-fig. 8.—Alimentary System. Aboral View with Aboral Gonad Removed. Abbreviations: AO, axial organ: AP, ampulla; D, diverticulum; FLI, first loop of intestine; FLS, first loop of stomach; G, gonad; INT, intestine; L, lantein: ML, mesenteric ligaments; OE, oesophagus; R, rectum; RAC, radial ambulacral canal; ST, stomach.

Bonnet, 1925), or “first curve of the intestine” (Cuénot, 1948). It continues horizontally in an anticlockwise direction (viewing the animal from the aboral pole) forming a dorso-ventral inflexion in each radius, to lead into the intestine (INT), without any sharp boundary, in interradius 2. This is contrary to the condition in Echinus, as described by Delage and Hérouard (1903), where the slight inflexions of the stomach are interradial. The condition in Evechinus is more similar to that described by Bonnet (1925) for Spharechinus and Paracentrotus, where the internal border of the stomach has a pentagonal aspect while the external border forms a five-rayed star the arms of which ascend in the radial zones along the walls of the test. The stomach of Heliocidaris erythrogramma also has this aspect and is of comparable diameter with that of Evechinus. In the interradial zones the gut is suspended in such a way that the external border falls to the interior of the internal border, held in place by strands of mesentery (ML) securing it to the perignathic girdle. The diameter of the stomach is of the order of 9 mm. At the junction of the stomach and the oesophagus is a sac-like dilatation, the diverticulum (D), which is said (Cuénot, 1948) to contain a feebly acidic liquid with a diastasic action on albumen and starch. He considers it the principal seat of absorption of the products of digestion. The end of the oesophagus and the beginning of the intestine are connected by a narrow, cilia-lined tube and with a diameter of between 1 and 2 mm. This is the siphon (SI), or accessory intestine. It lies on the inner side of the stomach and accompanies this organ through all its turns, to finally open into it once more at the base of the inflexion in radius II. It is attached to the stomach by a narrow strand of mesentery, while a further strand links the internal marginal canal (IMC) of the haemal system to its inner side. There is no sign of an accessory siphonal groove such as is found in Arbacia (Cuénot, 1948). Its function has been thought in the past to be the subservence of respiration, which it effects by keeping a stream of fresh water flowing through the gut, thus functioning in much the same way as the accessory intestine of certain worms. (Lang, 1896; Chadwick, 1900; MacBride, 1906). However, Bonnet (1925) has suggested that the siphon serves as a diversion channel for water taken in with the food, passing the water directly to the intestine in which the excrements accumulate, and thus permitting the concentration of diastases in the stomach. This view is supported by Cuénot (1948). It may be that the siphon fulfils both these functions. The internal epithelium of the siphon is well developed and thrown into numerous folds, so that, as Bonnet (1925), suggests “… on ne peut s'empecher de penser que cet organ ne joue pas seulement le rôle d'un conduit servant à laisser passer l'eau extérieure dans la seconde courbure de l'intestine, mais qu'il pourrait bien remplir aussi quelque autre fonction”. He gives, however, no suggestions as to what that function might be. The intestine (INT) is of larger diameter than the stomach (about 14 mm), but like the stomach, has very thin walls, which in fresh specimens appeal a pinkish-brown colour, in some places quite transparent. On the border between interradius 2 and radius III it turns upwards to return in the opposite direction to that of the stomach (i. e., clockwise, viewing the animal from the aboral pole). This inflexion, and the downward inflexion of the intestine in interradius 2, are held together by a continuous sheet of darkly pigmented mesentery which is continued along the whole internal border of the intestine. The intestine is even more convoluted than the stomach, being reflected down towards the lantern in each interradius, and upwards, overlying the corresponding loop of the stomach, in each radius (Text-fig. 8). This is in contrast to the condition in Echinus where, as described by MacBride (1906), the festoons of the intestine alternate with those of the stomach. In Heliocidaris erythrogramma the intestine follows the same course as in Evechinus but is of rather less diameter. Various names have also been applied to the intestine. Lang (1896) calls it the “superior spiral”, Delage and Hérouard

Text-fig. 9.—Alimentary System. Aboral View of Stomach with Intestine and Gonads Removed. Abbreviations: AP, ampulla; AU, auricula; D, diverticulum; FLI, first loop of intestine; FLS, first loop of stomach; IA, interambulacrum; IMC, internal marginal canal; L, lantern; ML, mesenteric ligament; OE, oesophagus; RAC, radial ambulacral canal; RLS, radial loop of stomach; SI, siphon. (1903) the “reflected canal”, Bonnet (1925) the “recurrent coil” and “dorsal or second flexure,” and Cuénot (1948) the “second curve of the intestine”. Arriving at the lateral border of radius III with interradius 3, the intestine is continued without any sharp boundary as the narrower rectum (R). This passes up between the previously mentioned radius and interradius, with gradually decreasing diameter, to the anus, which opens to the exterior from a variable position, usually a little excentric, within the periproct. A continuous sheet of mesentery runs along the entire internal border of the gut, becoming especially well developed and darkly pigmented along the intestine. On the external border the mesentery is represented by strong mesenteric ligaments (ML) which firmly attach the stomach to the perignathic girdle, the intestine to the overlying gonads, and both coils of the gut to the sides of the test. In all specimens examined the stomach has contained only a very small quantity of food materials in the process of digestion, while the intestine has always been more or less crammed with excrement in the form of small pellets. Many of the pellets appeared to be surrounded by a case of hyaline material, probably mucin. Lumps of this material were found at the beginning of the intestine in some specimens. It is probable that the internal epithelium cells in this region are specially concerned with the secretion of this material. The first turn of the alimentary canal is thought (Cuénot, 1948) to correspond to the larval intestine, while the second turn is the result of progressive elongation of the larval rectum.

Food of Evechinus chloroticus The great volume of the gut contents and the efficiency of the teeth in tearing and grinding food into small pieces, or fine powder in the case of hard objects, makes identification of such material difficult. However, animals have frequently been found holding pieces of the brown seaweed, Carpophyllum maschalocarpum, between their teeth, and all alimentary canals examined contained a large volume of such shredded brown alga, frequently together with green and red algae. The gut also contains much calcareous material, finely powdered and held together in the form of small balls. This would come from the shells of encrusting animals living on the seaweed and possibly also from animals such as tuberculous polychaetes encrusted on stones and rocks. In general it may be said that Evechinus chloroticus feeds on littoral seaweeds and their associated epifauna. One animal was found with a partially eaten leaf in its mouth of Coprosma repens, the common coastal “taupata”, the leaves of which are frequently found floating in the sea near the coast. This is interesting when taken together with Fell's (1952) account of ophiuroids, Pectinura maculata, in Dusky Sound feeding on pollen dropping from beeches (Nothofagus) overhanging the water. The only other record of such a nature is one he cites of a deep sea echinoid in the East Indies which feeds on the leaves of dicotyledonous trees washed out to sea by rivers. The fact that the exclusively marine echinoderms are able to feed on land angiosperms seems rather remarkable. In the case of Evechinus, however, such feeding is probably accidental, for in general the animal appears to exhibit little power of selection, eating almost anything edible with which it comes in contact. In the gut of one animal I found several small pieces of wood and in another a short length of string. Histology of the alimentary canal (Text-fig. 10, figs. 1–5) The digestive tube of Evechinus chloroticus consists of the four fundamental layers typical of all echinoids; the external or coelomic epithelium, a muscle layer, a layer of connective tissue, and an internal epithelium. The pharynx (Text-fig. 10, Fig. 1) is covered externally by a low ciliated epithelium (LEP) which is not continuous with the coelomic epithelium covering the rest of the gut, but forms the internal lining of the lantern coelom. It consists of small cells with conspicuous nuclei. Internal to this is a thin muscle layer of circular fibres (CM). This thin layer is in contrast to the description of Echinus by Bather (1900), where the pharynx is described as being a muscular organ. The connective tissue of the pharynx is very well developed. It appears to form two layers, one within the circular muscle and one outside it. The external layer is confined to the radii, where it forms five pronounced radial ridges (RR). It is to these ridges that the ligament bands uniting the pharynx to the jaws are attached. The connective tissue lying inside the muscle layer is present only in the interradii where it takes the form of five shallow triangles (ICT), the apices of which are directed internally. Bounding the lumen of the pharynx is a high, extensively folded, pseudo-stratified epithelium (IEP) containing numerous gland cells secreting mucin (MU). These are especially abundant in the five radii, in each of which the epithelium forms a deep fold. A thin cuticle (CU) invests the internal edge. Also observed in transverse sections of the pharynx are the five radial vessels (RHV) of the haemal system, running down the outside of each radial ridge. The cross section of the pharynx of Evechinus has a very different form from that of Echinus as figured by Chadwick (1900), where the outline is distinctly five-rayed and where there appear to be no radial ridges. The external epithelium (CEP) investing the oesophagu (Text-fig. 10, Figs. 2 and 3) is part of the coelomic epithelium covering the remainder of the alimentary canal which is also continued on the internal face of the test and is reflected on all the organs contained within the general body cavity. The cells appear similar

Text-fig. 10.—Histology of the Alimentary Canal. Fig. 1—T.S. pharynx. Fig. 2—T.S. oesophagus. Fig. 3—Portion of wall of oesophagus enlarged. Fig. 4—T.S. stomach. wall. Fig. 5—T.S. siphon. (All measurements expressed in millimetres. Figs. 1–2 drawn to the same scale.) Abbreviations: CEP, coclomic epithelium; CI, connective tissue: CM, circular muscle fibres; CU, cuticle; EMC, external marginal canal; ICT, interradial connective tissue; IEP, internal epithelium; IMC, internal marginal canal; LA, lacuna; LEP, epithelium of lantern coelom; LM, longitudinal muscle fibres; ME, mesentery; MM, muscle layer; MU, mucin; RHV, radial haemal canal; RR, radial ridge of connective tissue; SI, siphon; V, vacuole.

to those constituting the external epithelium of the pharynx. The muscle layer (MM) of the oesophagus is also relatively narrow but rather better developed than in the pharynx. There is an outer layer of circular fibres (CM), with longitudinal fibres (LM) also present, between the circular muscle and the connective tissue. In Echinus an opposite arrangement of the muscle layers of the oesophagus is described by Delage and Hérouard (1903). The connective tissue (CT) is not as thick in the oesophagus as it is in the pharynx and varies in width around the diameter of the oesophagus. It has an open mesh-like structure, forming lacunae (LA), in which the liquids of the haemal system circulate. The folded internal epithelium (IEP) is also of the pseudo-stratified type, as is the whole internal surface of the gut. It also contains numerous mucin-secreting (MU) cells, but these are not localized in any way like those of the pharynx. A strong cuticle (CU) is present on its inner border. The lacunal vessels accompanying the oesophagus (IMC, EMC) and the mesentery (ME) uniting it to the diverticulum and the apical pole are also revealed in cross section. The muscle layer of the stomach (Text-fig. 10, Fig. 4) is chiefly composed of longitudinal fibres (LM) although a few circular fibres are present. The connective tissue layer is of fairly uniform width with many large lacunal spaces (LA). The internal epithelium is high and extensively folded, with very short cilia present on its inner border. It is extremely vacuolated (V), but no stainable material could be detected in the vacuoles. This vacuolation, and the great development of the lacunae, are in accord with the function of the stomach, which is the main digesting and absorbing region of the alimentary canal. The histology of the diverticulum is very similar to that of the stomach, the main difference being in the cells of the internal epithelium, which in the diverticulum are crammed with a pink-staining granular material. There is no sign of any special glandular formation in the diverticulum, which, as Bannet points out, is a special glandular formation in the diverticulum, which, as Bonnet points out, is a junction of the oesophagus with the diverticulum a sharp transition could be seen from the dark-stained mucous-containing cells of the oesophagus to those of the diverticulum where practically no mucous is secreted. The muscle fibres surrounding the siphon (Text-fig. 10, Fig. 5) are mainly circular (CM), and some of them are continued up into the internal mesentery The connective tissue (CT) encloses large lacunae, and varies in width following the folds of the internal epithelium (IEP). This is also of the pseudo-stratified type but is not so vacuolated as that of the stomach, and has a conspicuous granular border. Very short cilia are present on the cells bounding the lumen. Cross sections of the stomach show the internal and external marginal canals excavated in connective tissue. The internal marginal canal (Text-fig. 10, Fig. 5, IMC) is also surrounded by circular muscle fibres and so is evidently contractile like the internal marginal canal of spatangids. The wall of the intestine is of much the same structure as the stomach, except that the connective tissue layer is thinner and contains fewer lacunae, while the internal epithelium is lower, less folded, and does not contain so many vacuoles. The rectum is rather more muscular than the intestine, with circular fibres quite well developed. Longitudinal muscle fibres are also present within the connective tissue of the internal mesentery. The connective tissue layer is thin, containing a few lacunae, while the internal epithelium is higher than in the intestine and quite extensively folded .The cells are rather more vacuolated than one would expect to find in a presumably inactive organ such as the rectum. Examination of the internal epithelium of the stomach and intestine is made especially difficult because of the great delicacy of this layer and the readiness with which it comes away from the underlying connective tissue. In collections of gut

contents much of the internal epithelium of the gut wall is found mixed with the food materials. In all references which I have consulted the internal epithelium of the entire alimentary canal of echinoids is described as being ciliated. I have experienced great difficulty in locating any cilia, except on the internal epithelium of the siphon and the stomach. Elsewhere they could not be identified with certainty, even when using an oil immersion objective. I would hesitate to say, therefore, that the entire interior surface of the alimentary canal is ciliated in Evechinus. Ambulacral System (Text-fig. 11; 12; 13; Fig. 5.) Dissection. The tube-feet lying in two broad zones on either side of each radius are the external indication of the ambulacral canals and ampullae passing up the internal surface of the test. In the central axis of the animal the water, or stone, canal can be seen passing from the madreporite to the top of the lantern, closely associated with the axial organ. The ambulacral ring and its appendages may be observed by the use of a hand lens. The water vascular, or ambulacral, system, corresponds to a ramifying coelomic cavity called the hydrocoel which is developed from the left anterior enterocoel of the larva. It serves the dual function of respiration and locomotion and is peculiar to the Echinodermata. The ambulacral system communicates with the exterior through the pores of the sieve-like madreporite (Text-fig. 12, Fig. 1). The minute canals (MC) leading from the pores are lined for the upper third of their length by a high epithelium possessing numerous long cilia, which gradually merges into a flat epithelium, also ciliated, though not so conspicuously. Occasionally the canals may be seen to branch but most lead straight down to the thin-walled collecting chamber, or madreporic ampulla (MAP) lying immediately beneath the madreporite. From the ampulla, the stone canal (Text-fig. 12, Fig. 2), closely adhering to the axial organ, leads down in the central axis of the animal to join the ambulacral ring, in interradius 2, at the top of the lantern (Text-fig. 12, Fig. 3, SC). It may be seen as a thin white strand on the wall of the axial organ to which it is joined by a sheet of connective tissue. The ambulacral ring (AR), as in all echinoids where a lantern is present, has been pushed up to lie at the top of the lantern, thus coming to encircle the oesophagus instead of the mouth. It is pentagonal in shape, with transparent walls, and lies close against the axial ends of the lantern ossicles (Text-fig. 12, figs. 3–5). In the five interradii short branches pass from the ring to the Polian vesicles (PV). These are small, slightly bilobed bodies, usuallv brown in colour, and often partially obscured by the compass elevator muscles (Text-fig. 12, figs. 3–4). They are lymphoid in nature. Their exact function is rather obscure but is probably the same as that of the brown gland—i.e., the elaboration of amoebocytes. Branches from the haemal ring also pass to them so that they form a link between the ambulacral and haemal systems. Most authors refer to them as “Polian vesicles”, apparently homologous with the paired structures of that name present in the Asteroidea. (Lang, 1896; Chadwick, 1900.) MacBride (1906), however, considers them to correspond instead to Tiedmann's bodies, as he regards the asteroid Polian vesicles chiefly as storehouses of fluid for the ambulacral system, while it is Tiedmann's bodies which are concerned solely with the production of amoebocytes. Cuénot (1948), on the other hand, does not commit himself to any homology but refers to them simply as “amas spongieux”. In the five radii, the radial ambulacral canals (RAC) are given off, which pass out horizontally beneath the rotulae of the lantern (Text-fig. 12, Fig. 4), make an angle of 90° at the edge of the lantern, and can be seen running down its outer

Text-fig. 11.—Ambulacral System. Fig. 1—Tube-foot from adoral surface. Fig. 2—L.S. tube-foot and ampulla. Fig. 3—T.S. tube-fot. Fig. 4—Adoral ampulla. Fig. 5—L.S. wall of ampulla enlarged. Fig. 6—Interior view of apical ambulacrum of a specimen 39 mm diameter. (All measurements expressed in millimetres.) Abbreviations: AP, ampulla; CEP, coelomic epithelium; CT, connective tissue; DP, pore-pairs; ECT, elastic connective tissue; EP, external epithelium; EXC, excurrent canal; ICF, internal cavity of tube-foot; IEP, internal epithelium; INC, incurrent canal; LT, lateral canal; LM, longitudinal muscle; LV, valve of lateral canal; M, madreporite; NP, podial nerve; NX, nerve plexus; OC, ocular plate; OCP, ocular pore; OP, single pore; RAC, radial ambulacral canal; SK, sucker; SP, spicules; T, test; TB, trabecula; WC, crystalline pigment; XL, xtalline plate.

surface, lying against the intermediate muscles between the jaws (Text-fig. 7, Fig. 7). After giving off branches to the buccal tube-feet belonging to that radius, each canal then passes under the archway formed by the fusion of the apophyses, and continues up the interior wall of the test (Text-fig. 13, Fig. 5), giving off numerous branches on either side of the ampullae of the tube-feet. It finally ends blindly by perforating the ocular plate in each radius in the form of a small sensory terminal tentacle. The ampullae (Text-fig. 11, Figs. 4–5) are the most obvious internal structures belonging to the ambulacral system. Because the tube-feet are numerous and crowded together, the ampullae are correspondingly numerous and flattened together, projecting like the leaves of a book into the cavity of the test. They are roughly mitre-shaped vesicles, 4 mm to 5 mm wide, and standing about 6 mm from the inner wall of the test, when the tube-feet are contracted as in preserved specimens. Their semi-transparent walls, on microscopic examination, are seen to have thin bands of muscle, or trabeculae (TB) passing across them, converging towards the base of each ampulla, where two openings lead to the lumen of the tube-foot. On the median side, near the base, the lateral canal (LT) from the radial vessel may be seen entering the ampulla. It is provided with a muscular valve (LV) which prevents any return of the ambulacral fluid back into the radial canal (Lang, 1896). In Echinus, as figured by both Chadwick (1900) and MacBride (1906), the pores leading to the tube-feet and the entrance of the lateral canal appear to be much closer together than in Evechinus, the median pore being practically continuous with that of the lateral canal Heliocidaris erythrogramma is similar to Evechinus in this respect. Although the ampullae of the aboral surface are principally respiratory structures and might be expected to be larger and more conspicuously thin-walled than those of the aboral region, they show no such differentiation. There is instead a gradation in size, the ampullae of the adoral surface becoming slightly larger as they approach the ambitus and then smaller again as the ambulacra ascend to the apex. Lang (1896) states that in young echinoids the tube-feet are connected with their ampullae by a single pore only. This may be so immediately after metamorphosis but in young specimens which I examined the only single pores present were those in the two rows of ambulacral plates immediately behind the ocular plates (Text-fig. 11, Fig. 6, OP). The tube-feet, or podia (Text-fig. 11, Figs. 1–3), lying in a double row in the five radii of the animal, are the only external structures associated with the ambulacral system. They are highly contractile organs, which may be extended twice as far as the length of the spines or contracted down to only a few millimetres in length. In this contracted condition they have an annulated appearance. They have no conspicuous coloration in preserved specimens, but in the living animal they are dark red, merging near the test to a pearly grey. When extended they look very attractive waving about in the water beyond the outer limit of the spines. The internal cavity (ICF) may be seen in whole mounts (Text-fig. 11, Fig. 1) as a less dense central area. A round sucker (SK) is present at the distal end, which is supported by a delicate calcareous plate, the xtalline plate (XL), sometimes called the “rosette” or “pellion” (Bather, 1900.) This is composed of 6 or 7 segments held together by a central calcareous ring, extending a little below the level of the rest of the plate. The outer edge of each segment is spiked with finger-like projections which are continued in about half way to the centre of the plate like supporting ribs. Other isolated spicules basically of the bihamate type (SP) may be seen scattered along the tube-foot, especially at its distal end Mortensen (1943) describes them as being rather scarce, but all tube-feet which I have examined have contained quite large numbers of them. He figures them as completely smooth C-shapes, but one to four small conical projections can be seen, usually at one or both ends and sometimes at the centre of the spicule (Text-fig. 3, Fig. 8).

In Evechinus chloroticus, as in most regular echinoids (Lang, 1896) there is very little evidence of polymorphism in the tube-feet. They are all of the simple “homiopod” type of Cuénot (1948). However, slight differences, quantitative rather than qualitative, may be seen between those on the adoral and aboral surfaces of the animal. Most conspicuously differentiated are the five pairs of buccal tube-feet which arise from small plates imbedded in the peristome. They are highly contractile structures which makes it difficult to obtain them as good mounts. The shape of the sucker with its supporting xtalline plate is oval instead of round. The plate has only four or five component segments but otherwise presents the same structure as that in the other tube-feet. Spicules are abundant and also a few large ones of irregular shapes (Text-fig. 3, Fig. 9). Internally it may be seen that these organs do not possess ampullae. This is probably because they function as gustatory organs (Lang, 1896, et al.) and so do not require the constant stream of water passing through them necessary for locomotion and respiration. The xtalline plate would thus appear to have no function, but remains as a merely vestigial structure. The remainder of the tube-feet on the oral surface are of the typical homiopod structure and function primarily as locomotory structures, although some respiratory exchange must take place. In the usual contracted condition seen in the laboratory they are about 7 mm long, but in the living animal they may be greatly extended, sometimes twice the length of the spines. The diameter of the disc is of the order of 1.5 mm. Locomotion is effected by contraction of the ampullae, which forces the liquid within them into the tube-feet, causing them to extend. The suckers are then able to take grip on fresh ground and haul along the rest of the body (MacBride, 1906). This type of locomotion, however, is only possible over hard surfaces such as the rocky bottoms on which Evechinus is most frequently found. On loose surfaces such as sand the spines take over the function of locomotion. On the aboral surface the tube-feet gradually become smaller and less muscular, finally ending in the small terminal tentacles at the top of each ambulacrum. Many are of the order of 3 mm in length, decreasing to about 1 mm at the apex, but these again may be extended far beyond the tips of the spines. The xtalline plate is present, but is less robust, with bigger spaces between its constituent spicules. The suckers of these tube-feet can seldom, if ever, be used for locomotion, but have instead a protective function, either against light or predators. (See section on Ecology and Behaviour.) Mortensen (1943) records Evechinus, like so many other littoral echinoids—e.g., Echinus miliaris (MacBride, 1906), covering itself with shells, stones, algae, etc, and I have often seen animals using their aboral tube-feet for this purpose. The force required to remove these protective coverings indicates that the suckers, though reduced in size, are still quite powerful. The principal function of the aboral tube-feet, however, is respiration, which is made easier by the comparative thinness of their walls. The ambulacral fluid flows through the lateral canals from the radial canal to the ampullae. The cilia lining each ampulla direct the fluid into the incurrent canal (Text-fig. 11, Figs. 2, 4, INC), that nearest to the radius, leading into the lumen of the tube-foot. Here a gaseous exchange is able to take place through the thin walls of the tube-foot. The oxygen-containing fluid is then transported back through the adradial excurrent canal (EXC) into the ampulla, where the oxygen is able to diffuse through its wall into the general body cavity. The fluid is said (Cuénot, 1948) to contain the same type of cells as are present in the coelomic fluid. Neither colourless amocbocytes, nor flagellates have been observed in Evechinus, but the radial canals and particularly the tips of the ampullae (Text-fig. 11, Fig. 4, WC), become pigmented by the deposition of echinochrome and yellow crystals of waste materials, indicating the presence of enhinochrome amoebocytes and munform cells.

The direction of flow through the madreporite was for some time problematical. It has, however, now been established by Cuénot (1948) that the current flows from the madreporite to the oral ring, thus maintaining the system in the state of turgidity necessary for its proper functioning. Histology of the ambulacral system The cells composing the walls of the organs and vessels of the ambulacral system are found in the four main layers typical of echinoids (Lang, 1896): (1) Outer epithelium. In the case of the tube-feet (Text-fig. 11, Fig. 3) this is part of the external, pseudo-stratified epithelium (EP) covering the whole external surface of the body. All the remaining organs and vessels are covered by the ciliated peritoneum of the body cavity (CEP). Only those parts which are actually within the body wall—i.e., the canals connecting the tube-feet to the ampullae—are devoid of this outer epithelium. (2) The connective tissue layer (CT) is very thin in most parts of the ambulacral system except in the tube-feet where it is well developed. Here there are no circular muscle fibres, their place being taken instead by strong elastic fibres (ECT) developed in the connective tissue. Spicules are often present within its meshes, notably in the tube-feet where they are very numerous, but a few may also be found in the ampullae. The xtalline plates of the tube-feet also lie within this connective tissue layer. (3) Muscle layer. This is lacking in all parts of the ambulacral system except the ampullae and the tube-feet. In the ampullae (Text-fig. 11, Figs. 4–5) it is a discontinuous layer in the form of parallel bands of muscle, or trabeculae (TB), passing through the lumen of the ampulla and converging towards the openings to the tube-feet. In this way the ampulla is enabled to have sufficient contractility to extend the tube-feet in locomotion and yet remain sufficiently thin-walled to play the major part in the animal's respiration. The tube-feet owe their contractility to a thick band of longitudinal muscle (Text-fig. 11, figs. 2–3, LM) which is continuous with that lining the canals through the test wall to the ampullae (Text-fig. 11, Fig. 2). (4) Internal epithelium. This is a low ciliated epithelium continuous throughout the whole system. In the stone canal (Text-fig. 12, Fig. 2), however, it is represented by a higher, pseudo-stratified epithelium (IEP), thrown into folds. It has numerous, long cilia (CIL). Chadwick (1900) describes the corresponding epithelium in Echinus as columnar. Mention must be made here of the nervous tissue connected with the ambulacral system. In transverse sections of the ambulacra (Text-Fig. 13, Fig. 5) the large radial nerve is very conspicuous lying below the radial ambulacral canal and separated from it by the pseudohaemal canal, but this will be described later in the section dealing with the nervous system. The large nerve supplying the tube-foot is also quite conspicuous in transverse sections of that organ, lying between the connective tissue layer and the external epithelium. The Axial Organ (Text-fig. 12, Figs. 1, 3; Text-fig. 13, Figs. 1–2.) Dissection. The axial organ may be seen, on opening the body cavity (Text-fig. 6), as a brown fusiform body suspended by mesentery in the central axis of the body. Its intimate relationships may only be determined by sectioning. The axial organ lies in the central axis of the body suspended by a mesentery from the apical pole and also having a close mesenteric attachment (Text-fig. 13, Fig. 1, ME) to the oesophagus. It is fusiform in shape, tapering at both ends to a thin strand. In mature specimens it is a brown colour, probably due to ramifications of the lacunal system over its periphery, and also to the deposition of pigment

Text-fig. 12.—Ambulacral and Lacunal Systems. Fig. 1—Decalcified madreporite. Fig. 2—T.S. stone canal. Fig. 3—Ambulacral ring and Polian vesicles. Fig. 4—L.S. upper portion of Aristotle's lantern. Fig. 5—T.S. ambulacral ring enlarged. (All measurements expressed in millimetres.) Abbreviations AB, amoebocvte; ABL, aboral lacunal ring; ABR, aboral ring sinus; AEP, internal epithelium of the axial sinus; AL, alveolus; AO, axial organ; AR, ambulacral ring; C, compass; CEM, compass elevator muscle; CEP, coelomic epithelium; CIL, cilia; CT, connective tissue; IEP, internal epithelium; IM, intermediate musle; LA, lacuna; LC, lantern coelom; LEP, epithelium lining lantern coelom; LL, long ligament of pharvnx; LT, lacunal ring; MAP, madieproic ampulla; MC, madrepoic canals, MM, muscle layer; OE, oesophagus; PG, pigment; PH pharvnx; PT, periproctal sinus; PV, Polian vesicle; R, rotula; RAC, radial ambulacral canal; RLC, roof of lantern coelom; SC, stone canal; TM, terminal sinus; TP, terminal process of axial organ; WC, crystals of waste.

within its tissues. In very young specimens, however, it is quite colourless. The stone canal (SC) may be seen as a narrow white strand passing up its side. Anatomically it is remarkably constant, and its structure in both Evechinus chloroticus and Heliocidaris erythrogramma is similar to that figured for Echinus esculentus (Chad-wick, 1900, et al.). The structure of the organ can only be revealed by histological examination (Text-fig. 13, Fig. 1). It is then seen as as a mass of tissue, kidney-shaped in cross-section, lying between the peritoneum of the body cavity and the epithelium of the axial sinus. However, the organ has become so large, especially in its central region, as to encroach considerably on the cavity of the axial sinus (AX), which then appears as the lumen of the gland. In the embryo, according to Delage and Hérouard (1903), the axial sinus may be seen to be quite distinct, with the axial organ represented by an island of mesenchyme, lying between the sinus and the main coelom. The organ has a typically lymphoid structure, consisting of numerous connective tissue fibres (FCT) stretched to form a fine mesh (CTM) on which the regularly shaped lymph cells are situated (Text-fig. 13, Fig. 2). This network is so dense as to form a solid mass in the peripheral region, or cortex (C), of the organ. Towards the centre, or medulla (MED), the network is much more open and small diverticula (DV) of the axial sinus may be seen projecting into it. The connective-tissue fibres in this central region are particularly well developed. Small branches from the main coelom also penetrate the organ in the peripheral region. They are given the name of canaliculae (CL) by Cuénot (1948). A layer of connective tissue (CT), continuous with the mesenteries supporting it, and also with that surrounding the stone canal, encloses the organ. In it may be seen numerous lacunae (LA), and here and there large masses of a yellow granular pigment (WC) which are thought to be deposited wastes. Masses of this pigment are also scattered throughout the gland, especially in the cortical region. Often it may be seen to have completely filled a lymph cell in the connective tissue network (Text-fig. 13, Fig. 2, WC). Amoebocytes formed by the lymph cells can also be seen scattered throughout the gland (Text-fig. 13, Fig. 2, AB). In the oral region (Text-fig. 12, Fig. 3), the gland is continued as a thin strand on to the lacunal ring surrounding the oesophagus. It is indistinguishable from the stone canal to the naked eye, but under a low power microscope may be seen to be distinct. Aborally it is continued in the form of a small terminal process (TP) beyond the axial sinus into a small cavity, the terminal sinus (TM), lying next to the madreporic ampulla (Text-fig. 12, Fig. 1). The function of the axial organ was for a long time obscure, to which the many names by which it has been known testifies. It has been variously described as the “heart”, “pseudo-heart”, “kidney”, “plastidogenic organ”, “lymph gland”, “dorsal organ”, “ovoid gland”, “brown gland” and “genital stolon” (Lang, 1896, et al).It may be seen from its typically lymphoid structure that its function is the elaboration of amoebocytes, and so the name of “lymph gland” would be quite appropriate. However, when Sarasins (1887), describing the organ in Asthenosoma, called it a nephridium, he was nearer the truth than at first sight appears, for the function of excretion in echinoids is performed by the phagocytic action of the amoebocytes produced by the axial organ. Also, wastes which are deposited in inactive crystalline form throughout the body are particularly abundant in the axial organ. The name “genital stolon” refers to the fact that the genital cells of the young echinoid arise in the axial organ and then migrate aborally to form a ring around the aboral sinus from which the gonads of the adult are derived. These relationships are discussed, however, in the section dealing with the reproductive system.

The axial organ thus appears to be almost identical histologically with the same organ in Echinus, although in cross-section it differs slightly, being kidney-shaped and rather diffuse, whereas it is rounded and more compact in Echinus. Chadwick (1900) describes the terminal process as perforating the madreporic ampulla in Echinus but this is not upheld by later workers (Cuénot, 1948). He also figures the axial sinus as extending down to lie next to the madreporic ampulla. In this I think he has mistaken the terminal sinus, as figured for the same animal by Cuénot, for the lower part of the axial sinus. Cuénot (1948) describes a connection between the axial organ and the stone canal at their apical extremities. This was not seen in Evechinus although the terminal part of the axial organ does become closely associated with the corresponding part of the stone canal. There seems to be some confusion as to whether the organ lies actually in or on the axial sinus. Lang (1896) and Chadwick (1900) both describe it as being surrounded by the sinus, but Cuénot (1948) describes the opposite condition—i.e., it surrounds the axial sinus. This certainly appears to be the condition in Evechinus. Coelomic Cavities (Text-figs. 6; 12, Fig. 1; Text-fig. 13, Figs. 1, 3, 5, 7.) The coelom of Evechinus is spacious, and like that of all echinoderms, notable for its subdivision into several smaller parts (Lang, 1896). Some of these have already been referred to in describing the organs with which they are associated. However, for uniformity, they will be briefly mentioned again here. All those cavities which have arisen from any of the enterocoelic vessels of the larva are considered by Lang (1896) to be coelomic. They are lined throughout by a squamous or cuboidal endothelium, which is usually ciliated. The most obvious division of the coelom is that constituting the general body cavity, which in Evechinus is almost filled by the large coils of the gut with its numerous folds, and by the massive gonads. About the central axis of the animal, however, there still remains quite a large amount of free space. The perforated mesenteries holding the gut and gonads in place incompletely partition the cavity. The peripharyngeal, or lantern, coelom has already been described in connection with the alimentary system. It is completely separated from the main coelom, and as in all gnathostomous echinoids, is of large size, containing within it the ossicles and muscles of Aristotle's lantern. Attention is drawn to the fact that it also has subdivisions: (1) The five pairs of branchiae, or external gills, in each interradius. (2) The five large dental pockets surrounding the soft part of each tooth at the top of the lantern. As already mentioned, its function is respiratory. The periproctal sinus (Text-fig. 12, Fig. 1, PT) surrounds the end of the rectum and at its lower edge is attached to the apical ossicles. A well defined ridge (Text-fig. 18, Fig. 7, PR) passes from ocular plates I and V on either side up to the madreporite, converging towards the genital pore in the shape of a V, and provides attachment for the limiting membrane of the sinus. A similar ridge is present on the apical plates of H. erythrogramma but is in the form of a discontinuous circle, being broken between plates. The wall of the sinus is formed from stout mesenteric sheets which have, however, a number of perforations communicating with the general body cavity. Immediately above the periproctal sinus is a small completely closed space, the perianal sinus (Text-fig. 13, Fig. 7, PA). The wall of this sinus is attached to the edge of the periproct surrounding the anus. It does not appear to be muscular in Evechinus. Also at the apical end of the animal is a sinus in the form of a ring (ABR) around the inner edge of the calyx. This is known as the aboral or ring sinus (Text-fig. 6, Fig. 1). The gonoducts pass through it without perforating its cavity and a haemal

strand (ABL) is to be found within its inner wall. An aboral nerve ring is also usually present in echinoids, but I was unable to demonstrate it in Evechinus chloroticus. The axial or glandular sinus has already been described in the section describing the axial organ (Text-fig. 13, Fig. 1, AX). In very young echinoids it communicates with the aboral ring sinus, but this communication is lost in the adult. (Lang, 1896, et al). The terminal sinus (TM), lying next to the madreporic ampulla and containing the terminal process of the axial organ, has also been previously described (Text-fig. 12, Fig. 1). The madreporic ampulla (MAP) is described by Lang (1896) as also being of enterocoelic origin and so may be regarded as a small branch of the coelom. As he points out, this provides an open communication between a closed division of the coelom and the water vascular system or hydrocoel (Text-fig. 12, Fig. 1). Two sets of radial divisions of the coelom also exist. They pass up the internal wall of the test in close connection with the radial nerves. The first of these are the epineural canals (Text-fig. 13, Figs. 3, 5, EC), which lie between the outer surface of the nerve and the test. Aborally they end blindly. In Echinus, they are said (Chad-wick, 1900) to thin out gradually as the mouth is approached, but in Evechinus there does not appear to be any decrease in diameter (Text-fig. 13, Fig. 3). A circular canal surrounds the mouth. The epineural vessels are considered homologous with the ambulacral grooves of asteroids, for in the Echinoidea as in the Holothuria and Ophiuroidea this groove has become roofed over to form a canal (Lang, 1896, et al.). The second set of radial canals are the pseudohaemal vessels (PC) sometimes known as perihaemal canals. They lie between the inner surface of the test and the radial ambulacral canal (Text-fig. 13, Figs. 3, 5), and have the same distribution as the epineural canals. They also possess an adoral, circular canal (PR). Their function is obscure. The ambulacral system may also be considered as a much ramified coelomic cavity. It is sometimes known as the hydrocoel and is derived from the left anterior enterocoel of the larva (Lang, 1896, et al.). It may be seen that the coelomic cavities of Evechinus conform to the typical echinoid pattern, and are almost identical with those of Echinus. The only differences are in small details—e.g., the well developed ridge on the apical plates providing attachment for the wall of the periproctal sinus is not described as being present in Echinus, while the muscles of the perianal sinus of Echinus could not be seen in Evechinus. Coelomic Fluid (Text-fig. 13.) The coelomic fluid of echinoids has been analysed and is said by Cuénot (1948) to consist principally of sea water in which traces of nitrogen as protein and in the form of urea and ammoniates are present. There is also less chlorine than in sea water, but the two fluids remain isotonic because of the non-electrolytes present in the coelomic fluid. Four types of cells are commonly present, and these have all been observed in Evechinus chloroticus and seem to correspond in nature and dimensions (Cuénot, 1948) to those found in Echinus esculentus. (1) Phagocytic amoebocytes (Text-fig. 13, Fig. 10). These are small hyaline bodies, 10μ to 15μ in length, which usually have several long, slender, lobose pseudo-podia by which they engulf food and make slow progression through the fluid. They are frequently vacuolated and also have granular inclusions within their cytoplasm, which Cuénot (1948) says are “sans doute des produits de déchet solides fabriqués ou phagocytés par eux”. He says they are chiefly phagocytic although they also have

Text-fig. 13.—Axial Organ, Nervous System and Coelomic Cavities. Fig. 1—T.S. axial organ. Fig. 2—T.S. axial organ enlarged. Fig. 3—L.S. Aristotle's lantern near mouth. Fig. 4—Perrpharyngeal nerve ring enlarged. Fig. 5 T.S. ambulacium. Fig. 6—L.S. edga of an alveolus. Fig. 7—Interior view of apical ossicles. Fig. 8—Flagellate from coelomic fluid. Fig. 9—Muriform bodies. Fig. 10—Amoeboryte from coelomic fluid. Fig. 11—Echinochrome bodies. (All measurements expressed in millimetres. Figs. 8–11 all drawn to the same scale.) Abbreviations: A, anus; AB, amoebocyte; AL, alveolus; AX, cavity of axial sinus; C, cortex; CEP, coelomic epithelium; CL, canaliculae; CT, connective tissue; CTM, connective tissue mesh; DV, diverticulum; EC, epineural canal; EPH, internal epithclium of the pharynx; FCT, fibrous connective tissue; GC, ganglion cells; GF, superficial ganglion; IM, intermediate muscle; L, ligament; LA, lacuna; LC, lateral canal; LEP, epithilum of lantern codon; LO, lamella of deoper oral system; M, mouth; M, madreporite; ME, mesentery; MED, medulla; MM, musch; NA, nerve to alveolus; NE, nerve fibres; NPD, pharyngeal nerve of deeper oral system; NPF, pharyngeal nerve of superficial system; PHN, nerve to pharynx; PPN, pedal and peripheral nerves; NR, pen-pharygeal nerve ring; P, peristome; PA, perianal sinus; PC, perihaemal canal; PR, perihaemal ring; PW, pseudopod before being withdrawn; RAC, radial ambulacral canal; RHV, radial haemal vessel; RN, radial nerve; RP, periproctal ridge; RR, radial ridge of connective tissue; SC, stone canal; SH, short ligment of pharynx; WC, waste crystals.

athrocytic properties—i.e., they also have an affinity for injected dyes and other introduced colouring materials. (2) Muriform cells (Text-fig. 13, Fig. 9). Amoebocytes, 10μ to 15μ in length, which move through the coelomic fluid by means of limax-type pseudopodia. There are numerous colourless granules included in their cytoplasm. They are neither phagocytic nor athrocytic but are thought by Cuénot (1948) to be protein reserves. (3) Echinochrome amoebocytes (Text-fig. 13, Fig. 11). These cells are very similar to the muriform bodies but are usually slightly larger (20μ) and contain the pigment echinochrome which gives them a deep brownish-red colour. This is apparently only present in the endoplasm, for when their pseudopodia are being withdrawn the colourless ectoplasm may be seen as a clear outer shell left behind when the dark red endoplasm has been withdrawn (PW). They also are neither phagocytic nor athrocytic. Cuénot (1948) notes that together with the muriform cells they are always abundant in parts of the animal which are in the process of formation or regeneration. (4) Flagellates (Text-fig. 13, Fig. 8). These are very numerous and may be seen swimming actively through the fluid by means of their very long single flagellum (35μ in length). The body is small and globular, approximately 5μ in diameter. Vacuoles and granular inclusions are occasionally seen in the usually hyaline protoplasm. It is not known whether they are parasitic Protozoa or cells elaborated by the animal itself. Cuénot (1948) says “on ne sait trop si ce sont des éléments normaux du liquide coelomique ou bien des Flagellés parasites du genre Oikomonas; le fait qu'on les trouve d'une façon constante chez les Cidaridés, les Réguliers et les Clypéastroides, en nombre toujours considérable, n'est pas favorable à l'hypothèse parasitaire”. The liquid fresh from the animal is a pale rose colour due to the dispersed echinochrome bodies it contains, but after standing for a short while a reddish coloured clot is formed floating on the surface. This is caused by the colourless amoebocytes uniting to form a network within the meshes of which the echino-chrome and muriform cells become entrapped. Cuénot (1948) points out that this pseudo-clot plays an important role in the healing of wounds. Some such mechanism is probably effective in other classes of Echinodermata for the group in general is described as having extensive powers of regeneration. Lacunal System (Text-fig. 6; 10, Figs. 2,4,5; Text-fig. 12, Figs. 1, 3–5; Text-fig. 13, Figs. 1, 3.) Dissection. The two marginal canals on the gut can be readily observed during the course of dissection as two thin white strands on either side of the oesophagus and stomach. Sometimes they may be quite red in colour, probably due to the presence of numerous echinochrome bodies within them. The collateral canal is quite transparent and readily seen floating above the lantern in the general body cavity. Sectioning is necessary to observe all other parts of the system. In the Echinodermata, particularly the Echinoidea and Holothuria, food materials are absorbed and transported from the gut to other parts of the body by a number of lacunae of varying size which collectively are known as the lacunal or haemal, system (Lang, 1896, et al.). The lacunae (LA) are merely spaces excavated within the connective tissues of the various organs and do not possess any true endothelial lining. Some are very small spaces, such as those developed within the wall of the intestine, while others again coalesce to form bundles of canals running together in definite directions, and may attain quite large size. Some may even have muscular walls. I have followed Bonnet's (1925) terminology in describing this system. He prefers to use the term canal rather than vessel for the larger aggregations of lacunae because it can be equally well applied to a single, microscopic lacuna. “Nous em-

ploierons, pour désigner les différentes portions de l'appareil absorbant, le terme de canal qui peut s'appliquer aussi bien aux conduits d'aspect vasculaire libre dans la cavité générale, qu'aux réseaux lacunaire de l'intestine.” The two main haemal canals lie on either side of the gut, the internal marginal canal (IMC) on the inner or axial border of the stomach and the external marginal canal (EMC) developed on its outer surface. They are also sometimes known as “ventral” and “dorsal” canals respectively, but these are not good terms when applied to the adults of the Eleutherozoa. It is best to reserve them for description of the larvae, as these alone possess a primary bilateral symmetry (Lang, 1896, et al.). Each canal arises at the top of the lantein, and passes down either side of the oesophagus, the external canal lying at the root of the mesentery uniting the oesophagus and the axial organ (Text-Fig. 6, EMC), while the internal canal lies on the opposite or free surface (Text-Fig. 6, IMC) Cuénot (1948), and Delage and Hérouard (1903) describe the internal canal alone as being present on the oesophagus Chadwick (1900), Lang (1896), and MacBride (1906), however, describe both canals as being present, which is the condition in Evechinus. In section, however (Text-Fig. 10, Fig. 2), the external marginal canal is seen to be little more than a strand of connective tissue, while the internal marginal canal is excavated as a true canal. At the junction between the oesophagus and stomach the internal canal passes on to the top of the siphon (Text-fig. 9; Text-fig. 10, Fig. 5), which it accompanies throughout its whole course, round the body and back to radius II. Here it passes on to the mesentery uniting the upward coil of the gut in that radius (Text-fig. 9), and finally loses its identity by breaking up into small lacunae on the internal mesentery of the intestine Similarly, the external canal passes from the oesophagus on to the diverticulum (Text-fig. 6), and thence to the external border of the stomach It is in general finer than the internal canal, and like it becomes lost on the surface of the intestine A network of small branching lacunae, excavated in the connective tissue of the stomach wall, pass between the two main canals They may be seen in transverse sections of the stomach (Text-fig. 10, Fig. 4), and to a lesser extent in the wall of the intestine. In radius II, the external marginal canal suddenly increases in diameter due to its junction with a large subsidiary vessel which runs parallel with it. This is the collateral canal (Text-Fig. 6, CL). It is independent of the gut and floats freely above the lantern in the general body cavity. At its two extremities—i.e., in interradu 3 and 1—it is continuous with the external canal, while in the interradu 4, 5 and 3 anastamoses connect the two canals It apparently functions as a reservoir for surplus liquid absorbed by the lacunae of the stomach wall. This canal is not present in all echinoids. Chadwick (1900) does not describe it in Echinus, but Bonnet (1925) says it does exist in that genus, and also in Psammechinus and Sphaerechinus, in all of which it follows an identical course with that of Evechinus. He did not find it present in Cidaris, Arbacia or Paracentrotus. I have found it present in Heliocidaris erythrogramma. The presence of a lacunal ring about the oesophagus at the top of the lantern is problematical in echinoids Cuénot (1948), Delage and Hérouard (1903) and MacBride (1906) all describe it as being present. “Elle entoure la base de l'oesophage, à coté du canal oral qu'elle suit exactement, placée un peu au-dessous et en dehors de lui” Chadwick (1900) says, “the existence of such a separate circular vessel is, however, open to doubt and this remark applies with greater force to the blood vessels which have been described as radiating from it, and traversing the ambulacra between the water-vascular and pseudohaemal canals. Such vessels are not evident in carefully prepared serial sections of the ambulacra”. I was not able to observe this circumoesophageal ring, nor its supposed branches to the Polian vesicles, even when examined under a binocular microscope. I had hoped to demonstrate it by injection of the lacunal system, but the attempts were unsuccessful. A lantern was then decalcified and sectioned to reveal the true nature of the haemal

ring (Text-fig. 12, Fig. 4). It was found that part of the wall of the wall of the ambulacral ring (AR) had a large mass of connective tissue (LT) adhering to it, in which numerous lacunae of varying sizes were developed. This, then, must correspond to the haemal ring described by Cuénot (1948) and others. In the interradii the lumen of the ambulacral ring becomes continuous with that of the Polian vesicles so that the strip of lacunal tissue is continuous with their wall, thus forming interradial branches from the haemal ring. From the lacunal ring five radial vessels descend the pharynx, lying outside the radial ridges of connective tissue. They may be seen in cross sections of that organ (Text-fig. 10, Fig. 1, RHV). Each is then said to cross the peristome and accompany the radial nerve and three radial canals up the five ambulacra, giving off lateral branches to the tube-feet, and finally ending in the terminal tentacle of the ocular plate. They are described as inserted between the water-vascular canal and perihaemal vessel. I have not been able to observe them in any sections which I have cut of the ambulacra, but in sections of the lantern at the level of the mouth (Text-fig. 13, Fig. 3) they could be seen passing from the side of the pharynx to join the radial vessels. Thus, though they are well developed near the mouth, they must gradually thin out to disappear completely by the time the ambital region is reached. The connective tissue surrounding the periphery of the axial organ has already been described as containing numerous lacunae, (Text-fig. 13, Fig. 1). From the adoral end of the gland a strand of connective tissue accompanies the stone canal and passes on to the lacunal tissue associated with the ambulacral ring (Text-fig. 12, Fig. 3). Aborally, a thin haemal strand is developed in the wall of the aboral ring sinus (Text-fig. 12, Fig. 1). The wall of the gonad also contains a lacunal plexus in most echinoids, but this is not well developed in Evechinus. The wall is very thin and the connective tissue little developed in all sections which I examined. The fluid circulating through the lacunae is similar to the coelomic fluid, but according to Lang (1896) contains much more albumen in solution. It contains the same kinds of cells floating within it. In most echinoids there is no muscle developed in the walls of the lacunae, so that there is only a slow displacement of the fluid, with no active circulation caused by contracting vessels. The axial organ was once thought to have such contractility and so was given its inappropriate names of “heart” and “pseudoheart”. The internal marginal canal of spatangids, however, has been seen (Cuénot, 1948) to exhibit irregular contractions, and this may also be the case in Evechinus. for in cross section the wall of the internal marginal canal is seen to be quite muscular. (Text-fig. 10, Fig. 5, CM). Nervous System (Text-fig. 13, figs. 3–6.) Dissection. The radial nerves are easily observed lying under the ambulacral canals passing up the test in each radius. The remainder of the nervous system, however, may only be observed by sectioning. The nervous system of echinoderms is now considered to consist of three independent systems: (1) the superficial oral system; (2) the deeper oral system; (3) the apical nervous system. In accord with the marked radial symmetry of the animal there is no central ganglionic mass or “brain” to co-ordinate movements. It might be expected that such a structure would develop in forms such as the irregular echinoids which have reverted to a tertiary bilateral symmetry, but evidently this is not so. MacBride (1906) has very neatly described the condition in echinoids: “In a dog the animal moves its legs, in a sea-urchin the legs move the animal”. Originally the superficial oral system, or ectoneural system, was the only one known (Lang, 1896), and in the Echinoidea it is certainly the most obvious system. The term “superficial” is used to describe it because in the Crinoidea and Astero-

idea it occupies an epithelial position throughout life. In all other echinoderms, however, it has sunk to a position below the body epithelium, and in echinoids it is to be found actually within the test wall, inside the body cavity (Lang, 1896, et al.). A central ring of nerve cells and fibres (NR) surrounds the mouth at the base of the lantern (Text-fig. 13, Figs. 3 and 4). Unlike the ambulacral ring, it has not become lifted up by the lantern and so occupies the same position in Evechinus as it does in agnathostomous sea-urchins. Ganglion cells (GC) form a dense, deeply-staining mass on its most adoral surface, while the remainder of the ring consists of nerve fibres (NF) among which a few scattered nerve cells may be seen. From the nerve ring in each radius nerves pass on to the pharynx (NPF) and gradually break up into a plexus, said to be still traceable on the wall of the stomach (Lang, 1896). A radial nerve trunk (RN) is also given off in each radius. It passes across the peristome, inserted between the perihaemal (aboral) and epineural (adoral) canals, to meet the radial ambulacral canals descending from the top of the lantern, so that all radial structures pass up the centre of each ambulacrum together (Text-fig. 13, Fig. 5, RN). It has the same histological structure as the nerve ring except that here the nerve fibres run longitudinally instead of being circular. Alternate branches (NPP) are given off to accompany the lateral canals of the ambulacral system to the tube-feet. These branches really consist of two nerves. One of them passes up the side of the tube-foot as the pedal nerve (Text-fig. 11, Fig. 3, NP), which is said to expand under the disc of each tube-foot as an intraepithelial ring (Lang, 1896). In this way the tube-feet must be able to function to some extent as tactile organs The other nerve is a peripheral nerve, and unites with the many other similar nerves to form a nerve plexus deep in the epithelium covering the exterior of the body. It is this nerve plexus which effects the movements of the sphaeridia, pedicellariae and spines. In sections of sphaeridia and spines a nerve ring may be seen passing around the base of each (Text-fig. 4, Figs. 7 and 8), while the sensory cushion of the gemmiform pedicellariae can be observed in sections of that organ (Text-fig. 4, Fig. 3). Cuénot (1948) describes the peripheral nerves of echinoids as often leaving a visible trace or groove on the test, which is particularly evident in species of Cidaris. No such grooves can be seen in Evechinus, but the wall of the most radial pore of each pore-pair is broken down at the point of emergence of each peripheral nerve (Text-fig. 1, Figs. 2 and 3). The deeper oral nervous system is only present in those echinoids which possess a lantern (Lang, 1896). It is usually in close association with the superficial system, so that macroscopically it is impossible to distinguish the two. However, high magnification reveals the nerves of the deeper system lying on the inner or axial border of superficial nerves (Text-fig. 13, Fig. 4). In the Ophiuroidea and Asteroidea there is a complete ring surrounding the oesophagus, but this is wanting in the Echinoidea and Holothuroidea (Lang, 1896). Instead, there are five lamellae (LO) consisting of both nerve cells and fibres which may be seen in Evechinus lying in close contact with the pharyngeal ring of the superficial system. From these lamellae nerves pass on to the pharynx in close association with the superficial nerves (NPD), and also on to the edges of the alveoli, up which they ascend (Text-fig. 13, Fig. 6, NA), and probably ramify to supply the jaw muscles. Delage and Hérouard (1903) claim that a short nerve typically passes back for a short distance lying on top of the radial trunks. However, this was not seen in Evechinus. An apical, or aboral, ring is described by most authors (Lang, 1896, et al.) as a delicate nerve trunk running within the wall of the aboral sinus Sectioning of the sinus, however, did not reveal it in Evechinus, although the use of specific nerve stains might possibly reveal it. There appears to be little deviation among echinoids from this basic pattern of nervous system. Even the irregular urchins, in which one might expect to find

Text-fig.. 14.—Reproductive System. Adoral View with Cut Removed. Abbreviations: ADC, adoral gonad; AO, axial organ; AP, ampulla; APG, coalesced apical mass of gonad; BG, minor branches of gonad; MES, mesentery; R, rectum; RAC, radial ambulacral canal. some central co-ordinating area developed, conform to it. Thus as Evechinus and Echinus are both regular urchins one would expect their respective nervous systems to be almost identical, and this seems, indeed, to be the case. Histological study of Heliocidaris erythrogramma was outside this investigation, so that apart from the ladial nerve trunks the nervous system was not examined, but it would probably be very similar to that of Evechinus chloroticus. Reproductive System (Text-fig. 6, 14, 15.) Dissection. To observe the full extent of the reproductive system of Evechinus chloroticus it is necessary to open the animal from the adoral surface and completely remove the alimentary canal. In ordinary dissection this drastic treatment is best saved for the last stage of the dissection, after the alimentary canal has been studied sufficiently and is no longer required. To observe the gonoduct within the gonad, it is necessary to slit carefully, with a scalpel, up the midline of the gland from the axial side, washing away the contents of the cut follicles as they ooze out and obscure the dissection. It is most easily observed in those animals where the duct is filled with the genital products, as it is then opaque, with patches of pigment showing up clearly along it. The reproductive organs (Text-fig. 15) are five large, arborescent glands lying in the interradii, extending from the apical pole down to the level of the lantern.

Text-fig. 15.—Reproductive System. Fig. 1—Genital plate No. 4 of a female. Fig. 2—Distal portion of the madreporite of a male. Fig. 3—Histology of mature female gonad. Fig. 4—Egg. Fig. 5—Ripe male follicle. Fig. 6—Histology of mature male gonad. Fig. 7—Developing gonad in a specimen of 19 mm d'imeter (All measurements expressed in millimetres. Figs. 1–2 drawn to the same scafe.) Abbreviations: FP, peritoneal epithelium; CT, connective tissue layer; F, follicle; FP, female papilla; FW, follicle wall; GD, gonoduct; GEP, germinal epithelium; GP, gonopore; IA, Interambulacral plate; M, madieporite; MM, muscle layer; MP, male papilla; MS, miliary spine; N, nucleus; NC, nutritive cell; NL. nucleolus; OG, Oogonium; OM, egg membrane; OP, ophicephalous pedicellaria; OY, oocyte; P, pigment; SS, secondary spine; ST, spermatids; SZ, spermatazoa; TF, trifoliate pedicellaria; Y, yolk.

In mature forms the five glands coalesce at their apical extremities (APG), remaining unjoined, however, in radius III. From this coalesced apical mass several minor branches (BG) may also arise, apparently at random, with no relation to the general symmetry of the animal. The gonad in either interradius 2 or 3 may be rather smaller than those in the other interradii. Mortensen (1943), surprisingly, describes the gonad as uncoalesced, but I can only assume that he dissected young specimens in which the gonads were still separate, or else specimens which had previously been starved, possibly by being kept without food in an aquarium tank for a few weeks. The great abundance of genital material is in contrast to the condition described for Echinus (Chadwick, 1900, et al.), where the gonads are restricted to the apical half of the body cavity, and where there is no coalescence between the neighbouring glands. The elongation of the glands in the interradii is similar to the condition in Sphaerechinus and Paracentrotus as described by Bonnet (1925). In Heliocidaris erythrogramma the gonads are also elongated dorso-ventrally. They are not, however, of such great volume and there is no apical coalescence. The gonoduct is quite conspicuous in this form and may be seen running down the length, of the gonad immediately below the inner or axial surface. In the matter of coalescence Evechinus more closely resembles Psammechinus, a genus closely allied to Echinus, in which the gonads are joined. The walls of the gonad consist of numerous follicles (F) within which the sexual cells are formed. Running through the centre of each gland is a narrow, thin-walled gonoduct (GD) which receives branches from the follicles all along its length. Aborally (Text-fig. 6) it may be seen passing from the gonad to penetrate the test in a pore (GP) at the outer edge of each of the four genital plates or in interradius 2, the madreporite. In a very young specimen of 19 mm diameter (Text-Fig. 15, Fig. 7), which I dissected, the gonads were just beginning to enlarge. All that could be seen of them at this stage was a central duct in each interradius along which small buds were just beginning to appear. This, together with the fact that the gonoduct has no specialized internal epithelium, suggests that the gonad has the structure of a large compound gland, with its follicles corresponding to numerous acini. The gonoduct is thus the first part of the gonad to make its appearance, and from it all other parts of the gland are derived. The colour of the gonads examined varied from pale yellow to bright orange and orange-brown, depending on the degree of ripeness. Pale yellow in most cases signified that the gonad was full of genital products ready to be discharged. The colour did not appear to have any relation to sex, as it does in some echinoids, including Echinus (Chadwick, 1900). The only difference noted was in the colour of the exuded stream of sexual products. In males this was a viscous, milky-white fluid, while in females it was pale yellow and rather less viscous. The genital cells of the Echinoidea, like those of the Asteroidea, Ophiuroidea and Crinoidea, are thought to arise in the axial organ and to migrate to form a ring, or genital strand, around the aboral sinus, from which the gonads of the adult are finally derived (Lang, 1896, et al.). This direct connection between the axial organ and the gonads apparently persists throughout life in the Asteroidea and Ophiuroidea, but in the Echinoidea it completely disappears in the adult. However, in a very small specimen of Evechinus chloroticus (diameter 19 mm) in which the gonads were just appearing as small buds from the central gonoduct in each interradius there was absolutely no sign of the genital strand. The genital strand must therefore, if present at all, atrophy at a very early stage in the development of the gonad. A narrow strip of strictly interradial mesentery runs down the midline of each gonad securing it to the test Other mesenteries, in the form of fine strands, pass from the alimentary canal on to the gonads and have already been described in connection with the alimentary system.

It is difficult to distinguish between the sexes by external characters. A genital papilla is present in both sexes (Text-fig. 15, Figs. 1 and 2). The male papilla (MP) is rather more than 1 mm in length and is dark in colour with a white tip. There was no sign of glistening white edges at the base of the papilla as described by Swann (1953) for Sphaerechinus granularis, Echinus esculentus, Paracentrotus lividus, Psammechinus microtuberculatus and P. miliaris, the four latter species all belonging to the Family Echinidae. The male papilla of Heliocidaris erythrogramma is much the same shape and dimensions as that of E. chloroticus. The female papilla (Text-fig. 15, Fig. 1, FM) is short and conical (0.5 mm to 0.6 mm in height), terminating in a circular aperture ringed round by dark pigment. The same appeared to be true of H. erythrogramma although the material examined was not well preserved. In the species described by Swann the female genital apertures are not borne on papillae, but are slightly sunk below the level of the rest of the plate. They are of no use for field identification of the sexes, however, because they are small and difficult to detect in among the spines and pedicellariae, even when examining under a low power binocular microscope. The genital pores are of about equal width (0.6 mm) in both males and females of Evechinus. This absence of any widening of the genital pores in the female is in agreement with the small egg size (0.1 mm). Also correlated with the small egg size, and consequent independence of the larvae, is the absence of any incubatory devices, such as are common in many Antarctic sea-urchins (Cuenot, 1948). It is surprising, however, that the genital pores of H. erythrogramma should also be like those of Evechinus, and show no widening in the female, as the eggs of that species are five times larger and undergo direct development (Mortensen, 1943). It appears that animals in which the gonads are ripe may be obtained throughout most of the year, except during the middle of winter, for ripe specimens were obtained as late as the middle of June. Early in August artificial fertilization of three specimens by injection of potassium chloride was unsuccessful, although the animals were obviously strongly stimulated, and during September several specimens were collected from Island Bay none of which contained eggs or sperms. However, on the 1st October a large ripe female was obtained from Makara. In July, 1953, two collecting trips were made to Oriental Bay, where specimens had been very abundant during the previous summer, but no Evechinus could be observed. This was also noted by B. E. Maxwell, who visited the area independently about the same time. It was thought, therefore, that the animals might only be present close inshore over the summer spawning period and might migrate out into deeper waters during the winter. However, over the same period in 1954, Evechinus was present in large numbers at Oriental Bay, while at Balena Bay throughout both 1953 and 1954 the number of animals present did not vary appreciably. There must, therefore, have been some other factor affecting the sea-urchins at Oriental Bay during the winter of 1953. The explanation mav even lie in a “kina” feast by some of the city's Maori population. Histology (Text-Fig. 15, figs. 3–6) The wall (FW) surrounding each follicle of the gonad is very thin, but nevertheless three definite layers may be distinguished. (1) An outer layer of small cuboidal cells with conspicuous nucler. This is the peritoneal epithelium (EP) which covers all organs within the general body cavity. (2) A thin muscle layer (MM). (3) A narrow strand of connective tissue (CT). Between the peritoneum and the muscle layer is a space, described by Cuénot (1948) as “plus ou moins virtuel”, in which migratory cells may occasionally be seen. The interior of each follicle is composed of a germinal epithelium (GEP). In animals where the gonad is not ripe the whole follicle is composed of the large

vesicular cells described by Cuénot (1948) which have a phagocytic action, engulfing those sperms and eggs left behind in the follicle after spawning. Globules of a yellow pigment (P) may be seen among them, and also new spermatagonia and oogonia beginning to develop. In a ripe male the spermatazoa (SZ) are shed into the centre of each follicle (Text-fig. 15, Fig. 5) where they accumulate as a dense mass which is visible to the naked eye in stained sections. The sperms are narrow, tapering bodies, about 6μ in length, and are produced in numbers which must be astronomical. Surrounding this central mass are germinal cells still in the process of spermatogenesis (ST). In those sections which I was able to examine this process was fairly far advanced, most of the cells containing spermatids. A few large masses of pigment may be seen throughout the epithelium. In the female (Text-Fig. 15, Fig. 3) the oogonia and oocytes are readily distinguished by virtue of their dark-staining yolk material. The oogonia (OG) are seen as small spherical cells in which the nucleus is still inconspicuous, lying among large, thin-walled nutritive cells (NC). The oocytes (OY) are much larger, with a conspicuous, vesicular nucleus (N) and large nucleolus (NL). The egg (Text-fig. 15, Fig. 4) is spherical, yellow brown in colour, and opaque with included yolk material. It has a diameter of 0.1 mm. Moore (1934), working on the biology of Echinus esculentus, points out that the gonad is the one organ of the body in which the animal can store reserve food material, and this is probably true also for Evechinus chloroticus. Pigment is abundant and other cell inclusions may well be nutritive. This is borne out by the fact that in two sea-urchins which had been kept in an aquarium tank practically without food for two weeks the gonads were extremely reduced in volume, so that only a thin strand remained in each interradius. During that time the animals had been moving actively about, so that they were evidently maintaining themselves by drawing on the food reserves present in the gonads. Parasitism No parasites have previously been recorded from Evechinus chloroticus, but during the course of this study three animals have been discovered associated with the echinoid. None appears to be actually parasitic, if one defines that term as the condition where the parasite does actual bodily harm to the host. Two are endocommensal animals living in the gut—namely, a protozoan, the ciliate Anophrys elongata Biggar and Weinrich, and a rhabdocoel which appears to be a previously undescribed species of the genus Syndesmis, of which the species S. echinorum (Francois, 1886) is commonly found in the gut of European echinoids. The members of the Family Umagillidae, to which the genus belongs, are commonly commensal in the gut of echinoderms, although this appears to be the first record of the family from the Southern Hemisphere. The animal is bright red in colour, with a whitish streak, probably corresponding to the alimentary canal, in the dorsal mid-line. It is concave ventrally, and slightly convex dorsally, with the anterior end rounded and the posterior end sharply pointed. It is of the order of 2 mm-2.5 mm in length and 0.5 mm-1.0 mm in width. It is hoped to give a full description of the animal at a later date. Although infestation was fairly general, it was not on the whole heavy, two or three being the usual number found, although one specimen examined contained 18. Another species of the same genus was also found in the gut of Heliocidaris erythrogramma and also appears to be previously undescribed. This rhabdocoel is 2 mm to 3 mm long and 1 mm to 1.5 mm wide, rather larger and more flattened than that found in the New Zealand echinoid. It was a whitish colour in preserved specimens, where, however, the natural coloration may have been lost. All the echinoids examined were heavily infested with this platyhelminth, one specimen

harbouring as many as 68, 20 of which were taken from the stomach. In Evechinus the intestine was the only part of the alimentary canal in which rhabdocoels were found. Although only one species of ciliate was identified from the gut of Evechinus, it is probable that using techniques similar to those employed by Powers (1935), when he investigated the ciliates of Tortugas echinoids, many more could be discovered. Such a project was outside the scope of this investigation, however. The ectocommensal animal living among the spines and pedicellariae of Evechinus is a small copepod which has yet to be positively identified. It may prove to be a common littoral copepod, such as Amphiascus littoralis (Stuckey, 1948), to which it appears superficially similar, which has extended its range to occupy an ecological niche similar to Echinocheres violaceus on Northern Hemisphere echinoids. That it is truly commensal is borne out by the fact that it is tolerated by the host and never grasped by its pedicellariae. Ecology and Behaviour As has already been mentioned, Evechinus chloroticus is to be found at Oriental and Balena Bays in large numbers, occurring in about knee-depth of water at low tide. Here the substratum is pebbly to stony, with the brown alga Carpophyllum maschalocarpum very abundant. This habitat is a little different from that described as typical for the species by Powell (1947), when he described the animal as being “found towards low tide in rock pools and crevices amongst seaweeds” Fell (1952) also describes it as being “especially a reef-dwelling, eulittoral form”, although he records it as tolerating a soft, muddy substrate, “the former Napier Inner Harbour before it was drained by the 1931 earthquake” However, at Island Bay, Wellington, which according to these authors would be an ideal habitat for the echinoid, Evechinus is quite scarce. Numerous trips to these collecting grounds have been made by members of the Zoology Department over the last two years, and at no time has Evechinus been found in large numbers, on several occasions not a single specimen having been seen. What has emerged from these observations, however, is the fact that Evechinus can apparently tolerate quite a wide range of littoral conditions. Most frequently the animals at Island Bay were found clinging to stones in positions where they were directly exposed to the ocean swell. Others again, however, were living in very different circumstances. Some were found in a high, shallow pool, which at low tide was completely isolated They were clinging to the sides, quite inconspicuous among the coralline seaweed. One was only about two inches below the water. Different conditions, again, are to be met with at Evans Bay, where the substratum is rather sandy but yet supports large numbers of Evechinus. It is therefore apparent that these animals are not limited to the rock pool environment, but are able to tolerate a wide range of littoral conditions. This tolerance is possibly one of the factors influencing the wide distribution of Evechinus chloroticus throughout the New Zealand region. Fell (1952) records fragments of a large individual taken by the “Alert” from Dusky Sound in 50 fathoms, but as he points out, this should not be treated as evidence that the species is able to live in such deep water. The way in which many littoral echinoids hold pebbles, empty mollusc shells, seaweed, etc., on the aboral portion of the test has been observed by numerous authors. It has been suggested by Mortensen (1943) to be in response to the greatly increased light intensity of littoral conditions, rather than for protective purposes This may be true of Evechinus, for although the animal has no specific light-sensitive organs it does appear to be responsive to light in some degree. When placed under strong light for photography the tube-feet soon became contracted However, in any group of the animals living at approximately the same depth on the collecting grounds, and therefore under uniform conditions of illumination, some would be

covered with numerous such articles, while others were quite bare. It has also been noticed in the laboratory that animals which had been removed from the aquarium tank for any purpose, almost completely covered themselves with pebbles when replaced. Very young specimens in particular hold on so many stones that they become almost buried. This, together with their small size and habit of living under stones, is a very effective protective device, making them extremely difficult to find. It would appear then, that although this habit may be to some extent a response to increased light intensity, it is also a protective response which is of positive value to the young echinoid. General Discussion Evechinus chloroticus is regarded by Mortensen (1943) as a primitive member of the Family Echinometridae, which he thinks represents a less specialized branch derived from the same evolutionary stem as the Family Echinidae. He is not certain of the common ancestor from which these two families have arisen, but thinks it would belong to either the stomopneustids or the phymosomatids. He considers that Evechinus shows primitive characters—namely, the regular hemispherical shape of the test; oligoporous ambulacral plates; and spines of simple structure and only moderate size. The possession of a single, strongly developed lateral tooth on the blades of the gemmiform pedicellariae is taken by Mortensen (1943) as the distinguishing character which places Evechinus in the Family Echinometridae, while the nature of the larval form and the paired nature of the poison glands are taken as supplementary evidence. This latter character he thinks must indicate a primitive condition, since the single glands found in all the Echinidae have double efferent ducts arising from them, thus giving evidence of an originally paired nature. In his key to the Family Echinometridae Mortensen (1943) describes the spicules of the tube-feet of Evechinus as “simply bihamate”. Although this superficially appears to be the case, when examined under high magnification small distal projections are seen to be present, rather similar to those described by Mortensen (1943) as present on the spicules of Selenchinus armatus, a form which he regards as closely related to Evechinus. I would suggest, therefore, that the spicules be deleted from the key, which is not invalidated, however, as the character is of supplementary value only. The apical system of Evechinus conforms to the general echinometrid pattern as described by Mortensen (1943). Attention is drawn to the fact that although he has described the pores of the ocular plates as unusually small they are actually quite wide, approximately of the same order of size as those he figures for Selenchinus armatus. The tendency, which is seen quite frequently in other members of the Echinoidea, for the porous surface of the madreporite to become extended on to the neighbouring genital plates (Lang, 1896), is described in Evechinus, and is apparently unusual in echinometrids, Anthocidaris crassispina being the only form in which such a condition had been noted by Mortensen (1943). He describes the madreporite of echinometrids as conspicuously enlarged, and this is certainly true for Evechinus, so that the tendency to extend the porous surface still further may indicate the continuance of an evolutionary trend. H. L. Clark (1912, 1925) takes a very different view of the systematic position and ancestry of Evechinus. He has proposed two alternative classifications of the genus, neither of which, however, corresponds to Mortensen's (1943). At first he regarded Evechinus as belonging to the Family Echinidae, of which it represented a specialized form, by virtue of its pseudo-polyporous ambulacral plates, apical system (oculars V and I insert) and its pedicellariae. However, later (1925), after Mortensen (1921) had published his description of the larval form, Clark apparently regarded the genus as too specialized to belong to the Echinidae, and accordingly, although it was only oligoporous, removed it to the Family Strongylocentrotidae,

a group of otherwise characteristically polyporous forms. This is not consistent with his general scheme of classification which is primarily based on the number of elements present in the ambulacral plates. This particular character has been disregarded by Mortensen (1943) as not of primary importance because he thinks multiplication of pores has arisen many times in separate lineages. Evechinus at least would bear this out, in that specimens have been found with occasional polyporous plates among the otherwise oligoporous ambulacral plates. H. L. Clark himself describes such an example. “I have pinned my fatih to the test structure, especially to the essential difference between the triporous and polyporous ambulacra and now I find that in a South African species (Paracentrotus agulhensis) the same ‘adult’ individual may show both kinds!… This is probably due to senescence, but it is perplexing nevertheless.” Although by using Mortensen's (1943) method of classification, which stresses the value of the pedicellariae, this problem is avoided. Nevertheless H. L. Clark (1925) points out an inconsistency in that Mortensen does not use it for the Temnopleuridae. “The conclusion which Dr. Mortensen reached for the Temnopleurids from his study of the pedicellariae of that family coincides with that which I have reached from the study of all the regular Echini—i.e., that while the pedicellariae often afford good specific characters they are not, taken by themselves, reliable as a guide in seeking for the true interrelationships of the species. It does not seem that a character of such uncertain value in the Temnopleuridae can possibly become of ‘prime’ importance in the closely related Echinidae.” The difficulties involved in settling the systematic position of Evechinus chloroticus are thus seen to involve such a wide group of genera as to be far outside the limits of this study to decide, concerned as it is chiefly with description of the anatomy and morphology of but the one form. However, investigation of the anatomy of Evechinus chloroticus, together with the rather superficial account of Heliocidaris erythrogramma, and comparison with Echinus esculentus, would seem to indicate the two former genera as belonging together in a group apart from Echinus and its allies. The alimentary canal of both Evechinus and H. erythrogramma is voluminous and greatly convoluted, much more so than is generally described in the Echinidae (Bonnet, 1925), particularly the genus Echinus (Chadwick, 1900). The stomach forms long vertical festoons in each radius, which are lacking in Echinus. They are described by Bonnet (1925) as being present in Paracentrotus, which, however, belongs to a different sub-family, the Sub-family Parechininae. The question is raised whether extensive coiling of the gut is a primitive character, upholding Mortensen's (1943) contention that the Echinometridae are less specialized than the Echinidae. The fact that the irregular echinoids which are generally considered high in the echinoderm evolutionary scale, have the alimentary canal following a comparatively simple course (Cuénot, 1948, et al), suggests that this may well be the case. The shape and structure of the pharynx of Evechinus is very different from that figured by Chadwick (1900) for Echinus, where there are apparently no large radial ridges of connective tissue such as can be seen in Evechinus. There are also small differences in the histology of the alimentary canal in the two forms. The pharynx is apparently more muscular in Echinus (Bather, 1900) than in Evechinus, and there is an opposite arrangement of longitudinal and circular muscle fibres in the oesophagus (Delage and Hérouard, 1903). Mortensen (1943) himself has drawn attention to the styloid processes borne by the epiphyses of the lantern, which are well developed in both H. erythrogramma and E. chloroticus. He describes them as lacking in the Echinidae, except for a slight indication in Echinus elegans, Psammechinus miliaris and Parechinus angulosus. However, it would be difficult to say whether the epiphysial processes of the echinometrids have been secondarily acquired or those of the echinids secondarily lost.

The apical connection between the axial organ and the stone canal, described by many authors as present in Echinus (Chadwick, 1900, et al.) does not appear to be developed in Evechinus. The well-defined ridge on the apical plates of both H. erythrogramma and E. chloroticus, for the attachment of the wall of the periproctal sinus, is not described for Echinus. With regard to the nervous system, the apical nerve ring and the radial nerves of the deeper oral system are either absent or only poorly developed in Evechinus. Otherwise the system is of the usual pattern described for echinoids (Lang, 1896, et al.). In both Evechinus and H. erythrogramma the lacunal system is quite well developed, although in Evechinus, and possibly in H. erythrogramma, the lacunal ring about the top of the oesophagus is of only microscopic size and the amount of haemal tissue in the walls of the gonads very slight. In Echinus esculentus these structures are described by some authors (Cuénot, 1948, et al.) as being well developed. Although the radial haemal canals are quite large where they run on the pharynx and near the mouth, they eventually taper away to nothing at the ambitus. However, this may also be the case with Echinus, as Chadwick (1900) found them impossible to detect in all sections which he cut of the ambulacra. A conspicuous collateral canal is present in both Evechinus and H. erythrogramma and follows the same course as it does in other echinoids from which it has been described (Bonnet, 1925). Of the members of the Echinidae which Bonnet investigated he found it to be present in Echinus and Psammechinus, but lacking in Paracentrotus. The internal marginal canal is muscular in Evechinus, which is apparently not the case in Echinus. The ambulacral system, axial organ and coelomic cavities are of the usual echinoid type (Lang, 1896,et al.) and show nothing remarkable. The tube-feet of the ambulacral system show the only small amount of differentiation characteristic of the regular echinoids (Cuénot, 1948). The great coalescence of the gonads in Evechinus is apparently not of taxonomic significance. Mortensen (1943) describes the condition as being fairly general in the Family Echinometridae, but those of Heliocidaris erythrogramma are quite small and remain distinct, although they are elongated to extend from the apex to the lantern in the same way as do those of Evechinus. In the Family Echinidae there is apparently the same inconsistency, for, although the gonads in general are described by Mortensen (1943) as remaining separate, those of Psammechinus and Parechinus are strongly coalesced, and they may apparently even become so in occasional specimens of Echinus esculentus. The appearance of the gonad in young specimens of Evechinus suggests that the gonoduct is the first part of the gonad to appear and that from it the follicles, in which the genital products are formed, are subsequently budded. It is possible that the gonads of all echinoids may arise in this way. The difference in colour of the mass of released sexual products, and the size of the genital papillae are the only characters by which the sexes of Evechinus can be distinguished, apart from microscopic identification of gonad samples. A similar difference in papillae between sexes has been noted in H. erythrogramma. The papillae of Evechinus and H. erythrogramma are almost identical, and differ in form from those of Echinus (Swann, 1953). There is no widening of the genital pores in the females of either Evechinus or H. erythrogramma. This is as would be expected in Evechinus where the eggs are small and the larvae of indirect development (Mortensen, 1921). However, in H erythrogramma the egg is five times larger than that of E. chloroticus and the development direct (Mortensen, 1921). So it is surprising that both male and female pores should be only of the same order of size as those of Evechinus.

From this discussion it may be seen that the anatomical differences between Evechinus, Heliocidaris and Echinus are very slight, bearing out H. L Clark's observation that “the relationship with each family is so close that it is impossible to fix a natural boundary, passing which no exceptions will be found”. However, such differences as there are appear to uphold the view that Evechinus and Heliocidaris should be placed together in a group apart from Echinus and its allies in the Echinidae, whether that group be as H. L. Clark (1925) proposes, the Family Strongylocentrotidae, or as Mortensen (1943) contends, the Family Echinometridae. References Bather, F. A, et al 1900. A Treatise on Zoology. ed. E. Ray Lankester. Pt. III. The Echinoderma London, Black. Bell, F. J., 1887. Description of a new species of Evechinus. Ann. Mag. Nat. Hist. XX (5) pp. 403–405. Bocquet, C., 1952. Copepodes semi-parasites et parasites des échinodermes de la région Roscoff. Description de Lichomolgus asterinae n.sp. Bull. Soc. Zool. France T. LXXVII, pp. 495–504 Bolles Lee A., 1950. The Microtomist's Vade-Mecum. 11th. ed. Lond, Churchill. Bonnet, A., 1925. Recherches sur l'appareil digestif et absorbant de quelques Echinides Réguliers. Ann. Inst. Océanog. T. II. Borradaile, L. A., et al. 1935. The Invertebrata; a Manual for the use of Students. 2nd. ed. Cambridge, C. U. P. Cuenot, L., 1948. Anatomie, éthologie et systématique des échinodermes. Traité de Zoologie, Anatomie, Systématique, Biologie. T. XI. ed. Pierre-P. Grassé. Paris. Masson et Cie. Chadwick, H. C., 1900. Echinus. Liverpool marine Biology Committee. L. M. B. C. Memoirs. III. Clark, H. L., 1912. Hawaiian and other Pacific Echini. Mem. Mus. Comp. Zool. Harvard. 34 (4) pp. 281–282. —1925. A catalogue of the recent Sea-urchins (Echinoidea) in the collection of the British Museum (Natural History), pp 133–134. Delage, Y. and Hérouard, E., 1903. Traité de Zoologie. T. III. Les Echinodermes. Paris, Reinwald. Farquhar, H., 1897. A contribution to the history of New Zealand echinoderms. Linn. Soc. Journ. Zool. 26, pp. 186–197. Fell, H. Barraclough., 1941 The direct development of a New Zealand Ophiuroid. Quart. Journ. Mic. Sct. 82 (3), pp 377–441. —1947. The constitution and relations of the New Zealand echinoderm fauna. N. Z. Sct. Cong. pp. 208–212. —1952. Echinoderms from southern New Zealand.Victoria. Univ. Coll. Zoo. pub. 18. —1953. The origin and migrations of the Australasian echinoderm faunas since the Mesozoic. Trans. Roy. Soc. N.Z. 81 (2). Francois, P. H., 1886. Sur le Syndesmis, nouveau type de Turbellariés décrit par W. A. Sillman.C. R. Acad. Sct. Paris. 103, pp. 752–754. Gordon, Isabella, 1926. The development of the calcareous test of Echinus miliaris. Phil. Trans. Roy. Soc. Lond. B, 214. VII, pp. 259–312. Hiraiwa, Y. K., 1932. On the dermal structure of a soft-shelled sea-urchin Asthenosoma ijimai Yoshiwara.J. Sci. Hiroshima Univ. Ser. B. 1 (1), pp. 65–80. Hunt, O. D., 1925. The food of the bottom fauna of the Plymouth fishing grounds.J. Mar. Biol. Assoc. 13 (3). Hyman, L. H., 1951. The Invertebrates. V.2. Platyhelminthes and Rhyncoela; the acoelomate Bilateria. New York, McGraw. Lang, A, 1896. Text-book of Comparative Anatomy. Trans. H. M. and M. Bernard. Part II. London, Macmillan. Lehman, H., 1946.Syndisyrinx franciscanus, an endoparasitic rhabdocoel of Strongylocentrotus. Biol. Bull. 91, pp. 295–311. Lovén, S., 1892. Echinologica.Bihang tell K. Svensk. Vet.-Akad. Handl. XVIII, Afd. 4. MacBride, E. W., 1906. Cambridge Natural History. V.I. Lond, Macmillan.

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