The Vascular and Nervous Systems of Struthiolaria (Prosobranchia, Mesogastropoda)

Transactions and Proceedings of the Royal Society of New Zealand, Volume 83, 1955-56, Page 721

The Vascular and Nervous Systems of Struthiolaria (Prosobranchia, Mesogastropoda) By J. E. Morton Department of Zoology Queen Mary College. University of London [Received by the Editor, January 7, 1954.] Abstract A detailed description is given of the arterial and venous systems, and of the nervous system of the mesogastropod prosobranch, Struthiolaria. Special attention is paid to modifications of the vascular system of functional importance in relation to the mode of life, and an account is presented of the arrangement of “phagocytic depots” in the neighborhood of the alimentary canal. The nervous system is compared with that of Aporrhais and Strombus, and its value and limitations as an indication of phylogeny are discussed. Most of the recent contributions to our knowledge of the anatomy of gastropods have been in the field of “functional morphology,” concentrating especially upon the pallial cavity, and the digestive and reproductive systems. Here a valuable picture has been built up of the relation of structure to function. The more favoured systems of the classical morphologists—the vascular and nervous—do not, however, lend themselves so easily to functional treatment, and have come to receive much less emphasis. Though the older accounts of these systems—often in minute detail—fill many pages in Bronn's Tierreichs and other textbooks, there have of recent years been few good accounts of a prosobranch nervous or blood system. One of the most well known is that of Crofts (1929) on Haliotis. Yet in soft-bodied animals like the Mollusca, in which the form of the body may be constantly changing, the distribution of blood in the haemocoele plays an important role in movements, and in changes of size and shape of the various organs. Especially in sedentary or slow-moving gastropods, which do not perform rapid muscular movements, we may think of the blood in the haemocoelic spaces as forming a “fluid skeleton”, and the pattern of the vascular system and the blood lacunae may have an adaptive importance entirely apart from the normal physiological role of the blood. It is therefore necessary to consider the “haemo-dynamics”, or the role played by the movements of large volumes of blood, for a full understanding of the mode of functioning of the various parts of the animal. In the same way, the trend of recent discussions on the nervous system has moved away from the classical studies of the relative positions and degree of concentration of the ganglia. As Fretter and Graham have pointed out (1949) there can be euthyneurous “streptoneura” and streptoneurous “euthyneura”; and it is in general realised that concentration of the nervous system must have taken place many times by parallel evolution in different groups. The usefulness of the nervous system as a character in major taxonomy is thus likely to be much more limited than was once thought. Functional studies on the nervous system are of course best advanced in the cephalopods, following upon the work of Young; but already studies of nervous control of muscular systems in gastropods,

especially in relation to the buccal mass and feeding movements. (Nisbet, 1950) are giving to the nervous system in this group a new source of interest. In the present paper it is proposed to continue the writer's recent accounts of the anatomy of Struthiolaria (reproductive system (1950), digestive system and pallial organs (1951)), and it is intended that this description of the vascular and nervous systems shall be read in the light of the previous work. For this reason, reference is made without detailed explanation to many of the structural and functional features that have already been described. Acknowledgment This work was carried out in 1946 in the Zoology Department of Auckland University College, while I was the holder of a temporary demonstratorship. I am grateful to Professor W. R. McGregor for first suggesting Struthiolaria as a subject of research. Also, I deeply appreciate the assistance given by Miss P. M. Ralph, of Victoria University College. I.—The Blood Vascular System In this account, special attention will be paid to modifications of the circulatory system in relation to the mode of life, keeping in mind the habits of Struthiolaria as a surface crawling and sand-burrowing mesogastropod, constructing respiratory and feeding tubes by the use of its proboscis, and collecting its food by the use of the ciliary and mucous systems of the pallial cavity The following special needs may be mentioned: (i) The rapid movement of large volumes of blood within the venous sinuses to secure the expansion of the foot and the erection of the proboscis. (See Morton (1951), p. 6.) (ii) The large distribution of blood to rapidly expansible regions such as the pallial skirt and the walls of the food groove. (iii) The large blood supply to regions performing specialised functions, such as the mucus-secreting areas of the foot and the pallial cavity, and the phagocytic depots of the digestive system. (See Morton (1951), p. 20.) (iv) The shifting of the respiratory circulation from the gill to the pallial skirt, correlated with the special development of a ctenidial food-collecting mechanism. Method Injections of the vascular system were made both with the coloured rubber latex preparation supplied by Turtox Biological Supply Houses, and with oil paints mixed with turpentine. Latex was found to be admirable for making solid preparations of larger vascular spaces, such as the principal venous sinuses; it was injected through the foot, and also into the arterial system through the ventricle, after the animal had been anaesthetised by slowly adding small quantities of 70% alcohol to sea-water. Latex was found less convenient for demonstrating the finer details of the peripheral parts of the vascular system, owing to the thin, rather inelastic character of the walls of the vessels. For this purpose turpentine-bound oil colours were employed, and were injected in several ways—through the ventricle, into the pedal sinus, or directly into the rectal sinus. Oil paint was found to provide a beautiful injecting mass, by which the smallest features of the peripheral vascular system could be shown up; it has the double

advantage of a low surface tension and fine penetrating power, combined with the absence of diffusion from the vessels in specimens preserved in alcohol. In the study of the visceral arterial supply, injection was usually not needed; the white deposits of calcium salts on the walls of the vessels enabled the smallest arteries to be accurately traced in dissection. The Heart and Pericardium There are few special features calling for remark in this region in Struthiolaria. The two-chambered heart is enclosed in a pericardial sac, lying at the base of the visceral mass on the left side, immediately behind the pallial cavity. The pericardium indents the left side of the large unpaired renal organ. A flattened squamous epithelium lines its cavity—a reminder of its coelomic origin. None of the blood vessels, however, has any squamous lining, though in some of the arteries the longitudinal connective tissue elements have a compact arrangement giving the appearance of a pseudo-epithelium. Nor are the arterial walls themselves muscular, though in a few cases strands of extrinsic muscle are inserted upon the tough collagenous wall of the vessel, as in the proboscideal artery (p. 17). The pericardial lining is never excretory in Struthiolaria, or—as far as is known—in any of the higher prosobranchs. This function is carried out by the renal organ and, to an increasing extent in the higher types, by the digestive gland. A reno-pericardial duct (Text-fig. 2, Rp) leads from the pericardium to the cavity of the renal sac, and this sac was itself originally one of a pair of tubular coelomoducts opening out of the pericardial coelom. The renopericardial Text-Fig. 1.—Stereogram of portion of the crystalline style sac and of the proximal region of the intestine, showing the location of the phagocytic depot, and its relation to the smaller vessels of the arterial supply. A St S, Anterior artery of the style sac. C. T. Connective tissue containing melanophores. Ext B W, External wall of visceral mass. Int A, Small arteries of intestinal wall. Int Ch, Intestinal channel of style sac. Phag. Phagocytic zone surrounding style sac. Phag 1, Appearance of phagocytic zone at surface of body wall. Phag 2, Phagocytic zone surrounding intestine. P Int, Proximal division of the Intestine. St Sac, Style sac. (See also Morton, J. E., 1951, p. 16.)

duct runs forward a short distance in the floor of the renal sac, into which it opens by a whitish papilla about a quarter of an inch behind the external renal aperture. The duct is about 50μ in diameter, its epithelium thrown into 5-8 longitudinal folds by differences in the height of the cells. All the cells bear very long cilia, reaching as much as 15μ in length, and their direction of beat seems to be outwards into the renal sac. In Struthioloria, neither the male nor the female retains the genito-pericardial duct, which was, however, reported by Fretter (1941) to survive in several genera of mesogastropods and stenoglossans. The auricle of the heart is thin-walled and pyriform; its broad anterior end receives the efferent ctenidial vein and a smaller, separately opening, left efferent renal vein. The wall of the auricle incorporates a thin mesh work of muscle fibres, capable of strong contractions. The tapered posterior end of the auricle opens by a valved aperture into the ventricle, which is a much more muscular sac. It is spongy in texture and is built up of a reticulum of interwoven fibres, Text-Fig. 2.—The renal organ opened along its right side to show the venous supply of the interior. The course of the left efferent renal vein is shown with broken lines, beneath the reflected left lobe of the renal organ. L Afft R V, Left afferent renal vein. L Efft R V, Left efferent renal vein. Pl R R L, Venous plexus of the right renal lobe. R Afft R V, Right afferent renal vein. R Ap, External renal aperture. R Efft R V, Right efferent renal vein. R L, Right lobe of the renal organ. Rm,. Rectum. R P, Reno-pericardial aperture and direction of its duct to pericardium. Subr, Point of origin of afferent renal veins from underlying subrenal sinus. Vas D, Coils of vas deferens. V Vdf, Venous plexus of the renal floor between the underlying coils of the vas deferens.

more coherent in the outer wall, and extending inwards to run in various directions across the lumen. The narrow apex of the ventricle opens behind into a structure known as the “truncus arteriosus”, which has been described in several other mesogastropods (see Moore, (1899)) and which forms virtually a third chamber of the heart, giving rise at either end to an arterial trunk. It is retropericardial in position, and bulges against the posterior side of the pericardium. Its walls are composed of non-contractile fibrous connective tissue. The Arterial System (Text-fig. 1, Text-fig. 2. Text-fig. 4) The arterial blood supply of Struthiolaria leaves the heart by two stout vessels, the anterior or cephalic aorta and the posterior or visceral aorta. These form continuations of the outer and the inner ends respectively of the truncus arteriosus. The anterior aorta passes forward through the haemocoele of the trunk to supply the organs of the head, foot, trunk and mantle. Near its origin it gives off a large branch to the style sac and the stomach. The remainder of the visceral mass draws its blood supply from the posterior aorta. 1. The Anterior Or Cephalic Aorta Turning forward from the truncus, the anterior aorta runs just beneath the pericardium, directly alongside the oesophagus, to enter the cavity of the trunk on the left side (Text-Fig. 4, Fig. I, A Ao 1) immediately above the columellar muscle. It continues forward for about half the distance to the head, close to the left side of the oesophagus, and then crosses obliquely above the oesophagus to run along the right side of the gut as far as the nerve ring, through which it passes immediately below the oesophagus. It then breaks up at a single point into the proboscideal (Pro A) and pedal arteries (Ped A) and gives rise in addition to several smaller branches. Before entering the trunk, the anterior aorta gives rise to several branches, of which the superficial gastric artery (S Gast A) is by far the largest, passing directly backwards along the left side of the visceral mass to branch extensively upon the dorsal wall of the style sac and stomach. About half way along the style sac. this vessel sends forward a more slender branch, the anterior artery of the style sac, (A St S), while the main trunk continues backwards over the stomach. The stomach in Struthiolaria forms a large flask-shaped sac from which the proximal limb of the intestine and the crystalline style sac lead forward together, being visible from the external surface as far as the posterior limit of the pericardium. This region of the alimentary canal is richly supplied with blood, and is invested by a zone of connective tissue, constituting a phagocytic depot, which is functionally a part of the digestive system. (Text-fig. 2.) Beneath the epithelium of the anterior part of the stomach, the connective tissue forms a dense meshwork, supporting huge numbers of amoebocytic cells or phagocytes of the same type as those found in the blood stream. This layer of phagocytes extends forward to surround the proximal limb of the intestine; here it becomes deeper than elsewhere and lies immediately beneath the epithelium. Around the style sac it is separated from direct contact with the base of the epithelium by a layer of much less dense connective tissue, filled with large, branching or spherical, black pigment cells. This zone of pigmented cells is clearly visible

Text-Fig. 3.—Schematic summary of the course of the venous system in Struthiolaria, and of the relationships of the principal veins and sinuses. externally, and forms a shield-shaped area overlying the whole exposed surface of the style sac, and sending back a salient on to the wall of the stomach. Around the pigmented zone, the investment of phagocytic connective tissue is cut across tangentially by the body wall, so that it appears as a narrow greyish-white line, delimiting the shield-shaped area of pigment. (Text-fig. 1, Phag 1) The pigmented zone is very richly vascularised from the superficial gastric artery and from its main branch, the anterior artery of the style sac. Both vessels contribute to a rich, closely anastomosing arterial plexus; and at its periphery, this plexus comes in close contact with the sheath of phagocytes. This is the locality in which these cells migrate in large numbers out of the blood stream and become aggregated in the connective tissue. Whether division or proliferation of phagocytic cells occurs here, or whether there is any reverse migration into the blood system, is still uncertain. The writer has already spoken of the role of the phagocytes in digestion (Morton, 1951). They probably form both an accessory ingesting system in the stomach and a means of excretion in the intestine. The phagocytic depot surrounding the intestine seems to merge directly into that of the stomach and the style sac, and it is apparently supplied also by several longitudinal intestinal arteries, running beneath the intestinal epithelium and arising from the posterior aorta near its origin from the heart. Similar depots

Text-Fig. 4.—Fig. 1—General view of the arterial system. The pallial cavity has been opened by the removal of its dorsal wall, bearing the gill and hypobranchial gland, and the visceral mass has been somewhat displaced from its normal relations. The haemocoele has been opened by a median longitudinal incision of the trunk, which has been carried forwards along the dorsal surface of the head and along the proboscis. The arteries of the foot are represented as by transparency. Fig. 2—The arterial system of the visceral mass and of the rectum and genital ducts (female), viewed from the right side. A Ao I, Point of entry of the anterior aorta into the trunk. A Gen D, Artery of the genital duct. A Mid I, Artery of the middle intestine. A Ped A, Anterior pedal artery. A Rect G A, Anterior rectogenital Artery. A Stg A, Anterior artery of the sorting area. A St S, Anterior artery of the style sac. Hep A, Smaller hepatic arteries. H Rect A, Hepatorectal artery. Integ A, Integumentary arteries. L Liv, Left lobe of the digestive gland. L Oes A, Longitudinal oesophageal artery. Oesoph, Oesophagus. Oes A 1, Oesophageal artery. Oes A 2, Oesophageal arteries within the trunk. Op A, Optic artery. Pall A, Pallial Artery. Ped A, Pedal artery. Pen A, Penial artery. Pro A, Proboscideal artery. P Stg A, Posterior Artery of the sorting area. P Subcut A, Posterior subcutaneous artery P. Teg Pro, Posterior tegumentary artery of the proboscis. P Tent A, Pallial tentacular Artery. Rect, Rectum. Rect A, Rectal branch of anterior rectogenital artery. R Fd G. A, Right artery of the food groove. R Liv, Right lobe of the digestive gland. S Gang A, Artery of the supraintestinal ganglion. S Gast A, Superficial gastric artery. Tent A, Tentacular artery. Visc A, Visceral artery or posterior aorta.

of phagocytic cells have been found to occur in Lunella smaragda (Turbinidae) in the connective tissue surrounding the posterior sorting caecum of the stomach (Morton—unpublished observation), and in numerous other prosobranchs. There seems frequently to be a close association of these phagocytes with the ciliated sorting areas of the stomach, which are usually placed in close vascular contact with the phagocytic depots. Thus, for example, in Struthiolaria, the greater part of the ciliary sorting area upon the dorsal wall of the stomach is supplied by the anterior artery of the sorting area (A Stg A), a branch of the superficial gastric artery, which runs along the posterior limit of the phagocytic depot, and gives off to the sorting area numerous close-set parallel branches, following the course of the ciliated grooves and ridges. From examination of sections the sorting area would appear to be seldom a region of dense storage of phagocytes; but it seems to be without doubt the site where the phagocytes may take up solid particles from the lumen of the stomach, probably after migrating between the epithelial cells. After the transparent veliger larvae of Struthiolaria had been fed with particles of finely divided neutral red, a coloured area was almost at once developed in this part of the wall of the stomach. A similar process of ingestion by phagocytes was observed in the posterior caecum of the stomach in Lunella smaragda; while an earlier description of ingestion by phagocytes in the sorting area is that of Yonge (1923) on the bivalve Mya. In Struthiolaria, the more posterior part of the sorting area is supplied from the left side by a series of small arteries similar to those from the anterior artery of the sorting area, but running in the opposite direction. These arteries arise as small, parallel branches from the posterior or visceral aorta. Shortly in front of the superficial gastric artery, the anterior aorta produces a series of three or four small oesophageal arteries, (Oes A 1), as well as a slender integumentary artery. (Integ A) supplying the external body wall adjacent to the pericardium. In the course of its passage through the trunk, the anterior aorta gives rise to the following series of arteries:— (i) Posterior integumentary artery of the trunk (P Integ A), a slender vessel crossing above the oesophagus to pass through the muscular body wall to the outer integument of the trunk. (ii) Oesophageal arteries (Oes A 2), a series of tiny vessels arising at regular intervals and breaking up finely upon the wall of the middle region of the oesophagus. Further forward, just behind the nerve ring, arises a slender longitudinal oesophageal artery (L Oes A) which runs backwards along the dorsal surface of the oesophagus as far as the posterior end of the trunk. (iii) Posterior subcutaneous artery (P Subcut A). This is a narrow but rather important vessel, springing from the aorta just after it crosses the oesophagus. It passes directly through the musculature of the dorsal wall of the trunk, to emerge on the outer surface, where it branches extensively just beneath the epithelium of the floor of the pallial cavity. Its most important branch is a prominent longitudinal artery, the right artery of the food groove (R Fd G. A) which runs backwards in the fold of epithelium that forms the right margin of this groove. This vessel conveys the main supply of blood to the wall of the food groove which—like all the mucus-secreting regions in Struthiolaria—is extremely well

vascularised. The rich supply of blood enables the great expansion in breadth of the bounding fold, which, together with the fold of the other side, can be extended across the food groove to close it as a temporary tube. (iv) Supraintestinal ganglionic artery (S Gang A). Some distance in front of the origin of (iii), a very slender vessel leaves the aorta on the left side, passes beneath the oesophagus and runs backwards along the left pleuroparietal connective. It supplies a fine plexus of blood vessels lying within the sheath of connective tissue surrounding the subintestinal ganglion which, like the nerve ring itself, is richly supplied with blood. Behind the ganglion, a very narrow artery continues backwards for some distance along the left parietovisceral connective. (v) Pallial Artery (Pall A). The supply of blood to the mantle skirt has taken on an increased importance in Struthiolaria. With the modification of the gill as an organ of food collection, the margin of the mantle has become the chief respiratory area; as well, its connective tissues are richly supplied with amoebocytes carrying the calcium salts required for shell secretion. The pallial artery is a stout trunk arising from the aorta on the left some distance behind the point where the oesophagus is crossed. It plunges through the muscles of the trunk at the level of the supra-intestinal ganglion. Within the mantle it divides into anterior and posterior branches which run around the skirt and become continuous again on the right to constitute a complete circum-pallial arterial loop. The pallial artery gives off numerous, close-set arteries which break up into fine parallel branches running towards the free margin of the pallial skirt. In addition, there are somewhat stouter branches running to the siphonal lappet on the left and to the pallial tentacle on the right (P Tent A). The edge of the mantle is very extensible, especially the posterior pallial skirt, which forms a thickened cushion over the calloused inner lip of the shell. Here the arterial vessels break up into a dense plexus from which blood is returned to the pallial venous loop. After passing through the nerve ring, below the oesophagus but above the short zygoneury (Pl. IV Zyg), the anterior aorta divides into the pedal and proboscideal arteries (Ped A, Pro A), both very stout trunks serving to pour a rapid supply of blood into the pedal and proboscideal sinuses respectively. At the same point, separate and paired tentacular (Tent A) and optic arteries (Op A) arise, running directly through the body wall to the cephalic tentacles and the optic peduncles. In the male, a very stout penial artery (Pen A) originates on the right side. It carries a large blood supply, by which the penis is able to be elongated and distended. The pedal artery continues as a stout median trunk which afer a short distance bifurcates into two lateral pedal arteries running backwards along the sides of the foot, deep to the sole. Each sends off, near its origin, a much shorter anterior pedal artery (A Ped A) which breaks up in the pedal gland along the anterior margin of the foot. The whole cavity of the foot consists of a large haemocoelic blood space, traversed from side to side by slender muscle strands and able to be quickly flooded with a large volume of blood. In the typical creeping movement by which the animal progresses, expansion of the foot is chiefly secured in this way. When the operculum is in use, for gaining purchase in the substratum (see

Morton, 1951), there is little or no blood in the pedal sinus; the foot is reduced to a narrow muscular column, and the sole diminished in size by the expulsion of blood into the cephalic sinus and the haemocoele of the trunk. The pedal arteries do not branch further and appear to ramify very little at their final distribution; blood is evidently poured into the pedal sinus through wide arterial trunks ending rather abruptly. The proboscideal artery produces branches of the same abruptly ending type. It passes forwards along the floor of the proboscis immediately below the oesophagus, and remains very wide. It carries a large volume of blood to the very extensible proboscis, which may be erected in the same way as a lamelli-branch siphon and is used as a piston for constructing the inhalant and exhalant tubes in the sand, and for periodically clearing them of obstruction. By extension of its longitudinal muscles, the proboscis may be elongated and pushed up to the surface of the substratum. As it is drawn back it is greatly widened so as to fashion the wall of the sand tube. By the flooding of its sinus with blood, the proboscis reaches three or four times its normal diameter, and sometimes increases to several times its normal length. Clearly its supply of blood is required to be rapid and efficient. This is in part assisted by the relaxing of the circular muscles of the wall of the proboscis, and is chiefly brought about by the action of slips of extrinsic muscle originating from the muscles of the wall of the proboscis, and inserted radially upon the connective tissue sheath of the proboscideal artery itself. The lumen of this vessel is thus able to be suddenly dilated, and blood is poured into the haemocoele of the proboscis from the termination of the proboscideal artery. Along its whole length, the proboscideal artery gives off also a series of tiny lateral arteries, which extend dorsally around the wall of the oesophagus and branch minutely. In addition there is a pair of stout posterior tegumentary arteries of the proboscis (P Teg Pro) arising about half way along the proboscis, and passing obliquely forwards and outwards to the muscular wall Reaching the buccal bulb, the proboscideal artery gives rise to a pair of slender salivary arteries running upwards to the dorsal surface of the bulb. Just behind the radular caecum, the main trunk bifurcates, and from each branch comes a series of arteries supplying the expansible integument of the oral disc. ii. The Posterior or Visceral Aorta The posterior aorta turns abruptly backwards from the inner end of the truncus arteriosus and plunges deeply between the style sac and the anterior lobe of the digestive gland Near its origin, two smaller arteries arise independently from the truncus. The first is the slender artery of the middle intestine (A Mid I) passing backwards and branching along the wall of the intestinal loop. The other is the anterior rectogenital artery (A Rect G. A) which crosses to the right below the floor of the renal organ and divides into two branches which appear on the surface of the visceral mass. The larger branch passes forward to supply the dorsal surface of the rectum (rectal artery) (Rect A); the smaller branch courses over the albumen gland and receptaculum seminis in the female, and over the vas deferens in the male, after which it too continues along the rectum (artery of the genital duct). From the posterior aorta itself, branches run to the deep aspect of the stomach, and there is also the branch previously described that forms the

Text-Fig. 5.—Fig. 3—General view of the venous system of the renal organ, ctenidium and pallial skirt. The right wall of the pallial cavity has been detached from its insertion on the columellai muscle, and has been reflected towards the observer to show the course of the rectal sinus. Fig. 4—Diagram to illustrate the relations of the principal venous sinuses. The two cephalopedal channels are represented as cut across in transverse section. A Abd S, Anterior Abdominal Sinus. A Br Pall. Tributaries of the pallial loop vein from the edge of the mantle skirt. Afft Ct V, Affcrent ctenidial vein. A Pall V, Anterior portion of the palhal loop vein. B Ct S, Basictenidial sinus. B Rect S, Branches of the rectal sinus. Ct Fil V, Blood spaces of the ctenidial filaments. Efft Ct V, Efferent ctenidial vein. Hypobr, Plexus of blood vessels of the hypobranchial gland. L Afft R V, Left afferent renal vein. L Ceph P, Left compartment of the cephalopedal sinus. L Efft R V, Left efferent renal vein. Neph Gl, “Nephridial gland.” P Br Pall, Tributaries from the posterior side of the pallial loop vein Ped S. Pedal sinus. Prob S, Prohoscideal sinus. R Afft R V, Right afferent renal veins R Ceph P. Right compartment of the cephalopedal sinus: R Efft R. V, Right eflerent renal veins. Rect PleX. Plexus of blood vessels on the wall of the rectum Rect S, Rectal sinus. S Osph S, Subosphradial sinus. Subct V. Subcutaneous veins. Subr S, Subrenal sinus. Tr L Eff. Tributaries of the left efferent renal vein. Visc S, Visceral sinus.

posterior artery of the sorting area. Through the anterior lobe of the digestive gland, a number of arteries run outwards from a deep origin, including a stout hepatorectal artery (H Rect A) giving rise finally to two superficial branches running forward along the rectum. Continuing backwards, the posterior aorta buries itself in the gonad, to emerge at the surface on the right side of the visceral spire. Here it runs along the concave surface to the tip of the spire. At regular intervals it produces deep branches supplying the digestive gland and the gonad, after which they come to the surface at the convex side of the spire and break up into small arterioles just beneath the thin integument. An important function of all the branches of the posterior aorta seems to be the storage of calcium salts: their walls are always conspicuous by their opaque, white coating, and they appear to play some role connected with the calcium metabolism of the adjacent digestive gland. Fretter (1943) has remarked upon a similar storage of calcium salts in the walls of the visceral arteries of Onchidella. The Venous System (Text-Fig. 5 and Text-fig. 3.) Struthiolaria has a well-developed system of closed veins which return blood from the peripheral lacunae to a system of central haemocoelic spaces. All the veins from the visceral mass drain into a narrow, spiral visceral sinus, while the greater part of the blood of the head, foot and trunk is returned to the much larger cephalopedal sinus. Both these spaces open in turn into a third cavity, the subrenal sinus, corresponding to the space called by Pelseneer (1906) the anterior abdominal sinus. This forms the principal collecting and distributing centre for the venous blood, which passes from it indirectly to the auricle, by way of the renal organ, the ctenidium or through the renal organ and the ctenidium together. The cephalopedal sinus is a wide space lying immediately upon the muscular floor of the trunk, and constituting the lower part of the trunk and occupying the whole floor of the haemocoele. It is formed by two narrowly separated and parallel sinuses, which may be referred to as the right and left cephalopedal spaces (Text-fig. 5, Fig. 4, R Ceph P, L Ceph P). The compartment on the right is arched in transverse section and is a good deal more spacious than the left, from which it is delimited by a thin, muscular septum passing obliquely across the cavity of the trunk. The left compartment is extremely shallow, forming a wide, dorsoventrally compressed cleft. On the floor of the cephalic cavity, just beneath the nerve ring, the right and left spaces of the cephalopedal sinus coalesce to form a single narrow channel which expands again in front into the pedal sinus (Ped S). The latter forms a very large cavity, occupying the whole of the foot, immediately above the muscles of the sole. It is very extensible and—when the foot is fully expanded, becomes flooded with blood from the pedal arteries. At the posterior end of the trunk, the two compartments of the cephalopedal sinus also converge, forming here a single, narrowly constricted opening into the subrenal sinus (Subr S). This consists of a shallow triangular space, lying beneath the posterior part of the floor of the mantle cavity and extending backwards below the pericardium, the renal organ and part of the gastric complex. Ventrally it is bounded by the posterior end of the columellar muscle as this attaches to the shell. In addition to blood from the cephalopedal sinus in front and

from the visceral sinus behind, the subrenal sinus appears to receive a smaller amount of venous blood which is conveyed backwards along the floor of the pallial cavity by the wide subcutaneous sinus of the food groove. From the subrenal sinus venous blood is distributed along one of two paths: by renal portal veins or afferent renal veins which open directly from it on the right dorsal aspect, or by the rectal sinus (Rect S), a stout vein running forward along the right side and conveying blood to the pallial roof and the gill. The visceral sinus (Visc S) runs forward to the subrenal collecting centre from the tip of the spire, being visible without dissection, along the concave side of the visceral mass. Upon either side it receives at right angles some 15-20 hepatogenital veins from the digestive gland and the gonad. These vessels alternate in position with the visceral arteries and meander across the viscera immediately beneath the integument. Further forward, on the left, several stout gastric veins return blood from the stomach. Vessels Returning Blood to the Cephalopedal Sinus The organs within the haemocoele of the trunk are directly bathed in venous blood, which seeps downward through the spongy connective tissue surrounding the oesophagus, to the cephalopedal channels along the floor of the cavity. From the head, the integument of the foot and the pallial region, an extensive system of closed veins returns blood to the sinuses of the trunk, while a very large volume of blood is returned directly from the haemocoele of the proboscis which constitutes the proboscideal sinus (Prob S). This space is especially wide distally in the region of the eversible oral disc, while in addition there is a finely branching system of subcutaneous rostral veins returning blood from the integument of this region. The closed veins from the head, trunk and mantle include the following:— (i) Tentacular veins, running inwards to the base of the proboscideal sinus, where they are joined by a pair of short, wide optic veins. (ii) Penial vein, a stout trunk traversing the cephalic penis in the male and opening close to the right tentacular vein. (iii) Small subcutaneous veins (Text-Fig. 5, Fig. 3, Subct V) are everywhere present just beneath the external integument of the body wall, forming a closely spaced system at the sides of the foot and trunk. (iv) Circumpallial venous system. A stout pallial vein opens into the left cephalopedal channel just beside the supraintestinal ganglion. It returns blood from a single pallial loop vessel (A Pall V) which passes right round the mantle but opens through the body wall only at the single point upon the left side. On the anterior side of the loop vein the mantle is very extensible and blood is drained back from its edge by a number of rather stout pallial veins opening into the loop. From its posterior side the pallial vein receives blood from the external integument only, by a series of long, narrow veins coursing forward parallel to each other (P Br Pall). All the blood from the deeper aspects of the mantle is returned to the venous network of the hypobranchial gland and is conveyed to the ctenidium. Course of the Blood Leaving the Subrenal Sinus In the Gastropoda, the collected venous blood, before being returned to the heart, becomes side-tracked in varying proportions through the renal or ctenidial

Text-fig. 6.—Fig. 5—General diagram of the nervous system from the dorsal aspect. The buccal nerves and portions of the left and right parieto-visceral connectives have been omitted. C Ao, Anterior or cephalic aorta. Ce G, Cerebral ganglia. C Teg N, Cephalic tegumentary nerves. C Tent N, Cephalic tentacular nerves. Ct N, Ctenidial nerve. Fil N, Nerves to the gill filaments. L Dial, Left Dialyneury. L Pall N, Left Pallial nerve. L Pa-Visc C, Left parieto-visceral connective. L Pl-Pa C, Left pleuroparietal connective. L Visc G, Left visceral ganglion. N Bw, Nerve to the body wall from the right pleural ganglion. Oes, Oesophagus. Oe N. Oesophageal nerve. Op N, Optic nerve. Osph, Osphradium with underlying ganglionic trunk. Oto N, Otocystic nerve. P Bu, Nerves of the proboscideo-buccal series. Ped G, Pedal ganglia. Ped N Lat, Lateral pedal nerves. Rect N, Rectal nerve. Ren N, Renal nerve. R Pa, Right pallial nerve. R Pa-Visc C, Right parieto-visceral connective. R Visc G, Right visceral ganglion. Sb G, Subintestinal ganglion. S Int G, Supraintestinal ganglion. Siph N. Siphonal nerve. Visc Comm, Visceral commissure. Visc N. Visceral nerve.

systems, or in some cases through both together. In the more archaic prosobranchs, the blood was primitively carried forward from the subrenal sinus to the pallial roof for re-oxygenation, by two symmetrical veins, of which only one survives in the Monotocardia, forming the longitudinal rectal sinus of the right side (see Pelseneer (1906), p. 100). A large part of the blood, however, passes through a renal portal system into the “kidney”, and this is the case even in such archaic forms as Haliotis (Crofts, 1929), in which by far the larger volume of the venous blood goes to the right renal organ. The rectal sinus of the right side, opening directly into the basibranchial sinus, is here very small. The efferent renal vein of the right side may either join the rectal sinus, or enter into the venous anastomosis in the hypobranchial gland. In a few cases, it carries blood directly to the afferent ctenidial vein. There are four paths by which—in Struthiolaria—the blood from the subrenal sinus ultimately reaches the auricle. As compared with Dakin's account of Buccinum (1912) it will be seen that route (1) is relatively much more important in Struthioloria as compared with route (3). The by-passing in whole or in part of the renal organ appears to be a character of specialised monotocardians; and in Struthiolaria and other ciliary feeders, where the gill is extremely large, there is a functional explanation for a large direct supply of ctenidial blood, independent of blood from the renal organ. (i) Through the ctenidial system alone, blood being conveyed forward by the rectal sinus, through the anastomosis of the hypobranchial gland, and thence to the ctenidium, returning by the efferent ctenidial vein to the auricle. (ii) Through the renal organ alone, by way of the left renal portal vein and passing directly to the auricle through the left efferent renal vein. (iii) Through both the renal organ and the ctenidial system, a smaller portion of renal blood of the right side being collected by the right efferent renal vein and carried forward by way of the hypobranchial anastomosis to the ctenidim. (iv) Through the mantle alone, a very small amount of blood from the mantle and osphradium draining directly into the efferent ctenidial vein and passing at once to the auricle. The Blood Vessels of the Renal Organ The subrenal sinus in Struthiolaria lies immediately beneath the floor of the renal organ, and the afferent renal or renal portal veins are in consequence very short. The lining of the renal sac is infolded to form excretory filaments on two aspects only, and the organ has a bilobed structure, with a left excretory lobe forming its roof and a smaller right lobe forming portion of the floor (Text-fig. 2, R. L). The two lobes are completely separated on the right side by the passage of the rectum and the blood entering the renal organ from the subrenal sinus is at once distributed along one or the other of two distinct systems. The left afferent renal vein at first proceeds forwards from the subrenal sinus along the anterior part of the floor of the renal sac. It then loops upwards to run back along the whole length of the roof, supplying the filaments of the left excretory lobe over their internal surface. At close and fairly regular intervals, secondary and tertiary afferent vessels are given off at right angles, and the whole system so formed traverses the internal wall of the renal organ, each vessel running along the summit of a renal filament of the corresponding order. This

system of filaments projects deeply into the cavity of the renal sac, and it would appear in general to be very characteristic of the renal organ of prosobranchs that afferent blood is distributed to the renal filaments by vessels arriving along their summits. All the blood from the left renal lobe is returned directly to the heart. Blood received at the summit passes through the narrow, cleft-like space between the two sheets of epithelium in each filament. It reaches in this way a series of basal sinuses lying beneath the filaments close to the external wall of the renal organ. These sinuses open directly into a set of 15-20 collecting vessels, (Tr L Eff) which pass across the roof of the renal sac from right to left, to open at right angles into the wide left efferent renal vein (L Efft R V) which runs along the pericardial side of the renal organ. This vessel opens separately into the anterior end of the auricle at some distance from the efferent ctenidial vein. The second—and smaller—portion of renal blood from the subrenal sinus is distributed through the filaments of the right excretory lobe. There is no single afferent trunk, but a series of vessels which radiate from a common centre through a reticulate mesh of renal filaments. These filaments of the right lobe are not arranged as are those of the left in a regular, monopodial branching pattern, and moreover the afferent vessels do not proceed along the summits. The general direction of blood flow is not vertical from summit to base, but transverse, from right to left, through the spaces within the filaments. The blood from the right lobe of the renal organ is eventually taken into a larger collecting vessel which we may call the right efferent renal vein (R Efft R V). This continues forward along the ventral side of the rectum. In front of the renal organ it breaks up into a close-set plexus over the surface of the rectum, and this becomes continuous with the venous reticulum formed by the branches of the rectal sinus. The blood from the right lobe of the kidney thus joins the rest of the blood destined for the gill and is effectively separated from that of the left lobe, which returns directly to the auricle. The Ctenidial Blood Supply The great bulk of the ctenidial blood in Struthiolaria is carried directly from the subrenal sinus, by a large rectal sinus (Text-fig. 5, Fig. 3, Rect S) which is visible through the external body wall throughout its length. It leaves the right anterior corner of the subrenal sinus and runs forward along the pallial wall, a short distance below the rectum and the genital duct. On its ventral side it receives along its course a little additional blood from the body wall, and from its dorsal side it gives off at right angles a long series of parallel branches running across a narrow strip of the mantle to reach the surface of the rectum. Here, along the right side, that is to say—upon the externally visible wall of the rectum, is located a fine, close-set plexus of tiny vessels, formed by the anastomosing of the ultimate venous branches from the rectal sinus (Rect PleX). Into the same plexus flows also the blood from the right efferent renal vein. Along the left aspect of the rectum, the blood is again collected by a series of small, transverse vessels which carry it through the subepithelial connective tissue of the hypobranchial gland to the afferent ctenidial vein (Afft Ct V). The hypobranchial gland occupies the whole of the roof of the mantle, between the rectum and the ctenidium. It is very liberally supplied with blood, as are all the mucus-secreting regions in Struthiolaria. The transverse vessels from the rectum anastomose freely, and supply a fine reticulum of small venules, while the

chief of these vessels continue more or less directly across the hypobranchial gland to the ctenidium. As it leaves the hypobranchial gland, the blood is drained towards the left into the afferent ctenidial vein, a longitudinal collecting vessel which runs along the right side of the ctenidium, and from which venous blood passes directly into the long, narrow blood spaces of the ctenidial filaments (Ct Fil V). The interior of each filament constitutes a single, undivided blood channel. Some amount of re-oxygenation may take place through the membranous, non-ciliated wall of the attached part of the filament; in the stiff rod-like extensions of the filaments, however, the blood space lies for the most part between a pair of chitinous skeletal rods, underlying the epithelium of the filament. The greater part of respiration is evidently pallial (see above p. —). After traversing the filament to the left side of the gill, blood is received into the basictenidial sinus, (B Ct S) which is a long shallow space lying in the pallial roof. It overlies the separate gill filaments for about one-third of their attached length, and over this distance the separate filamentar blood spaces are widely open to the collecting sinus. The stout efferent ctenidial vein (Efft Ct V) passes along the left or axial edge of the ctenidium, and receives blood through a series of wide apertures from the basibranchial smus. The vein then passes through the posterior wall of the pallial cavity into the pericardium where it opens through the anterior end of the auricle. From the left aspect of the mantle, immediately alongside the gill, a small amount of blood is returned directly to the efferent ctenidial vein, while in addition a narrow subosphradial sinus (S Osph S) communicates with that vessel by a series of short cross connections. This portion of the pallial blood thus returns directly to the heart without diversion through either the renal organ or the ctenidium. II. The Nervous System (Text-figs. 6 and 7.) On the nervous system in the Struthiolariidae we have no previous information at all with the exception of a short description by Bouvier for Pelicaria vermis. Bouvier's figure, reproduced at p. 339 of Simroth's volume in Bronn's Tierreichs, portrays the nerve ring very accurately, with the exception of the pedal ganglia which were evidently omitted from his dissection (see Simroth, (1907)). The perioesophageal nerve ring consists of the three typical pairs of cerebral, pleural and pedal ganglia, as well as a seventh ganglion, the subintestinal, which has become secondarily incorporated in the ring. This ganglion is morphologically a part of the visceral nervous system which consists essentially of long paired trunks, connecting with the nerve ring the two sets of distal ganglia, the paired parietal ganglia and the visceral or abdominal ganglia. In the chiastoneury which followed upon torsion, the morphologically right parietal ganglion has crossed over the oesophagus to the left to form the supraintestinal ganglion, and the originally left parietal member has passed below the gut as the topographically right subintestinal ganglion. The Perioesophageal Nerve Ring. The cerebral ganglia are approximately kidney-shaped, half a millimetre across and set close together in the midline by their convex edge. They lie on the dorsal surface of the oesophagus, and on either side a cerebropedal connective passes obliquely forward to meet the corresponding pedal ganglion at its posterior end. Each cerebral ganglion (Text-fig. 7, Ce G)

Text-Fig. 7.—Fig. 6—Lateral view of the perioesophageal nerve ring (left half removed) and the nerve supply of the proboscis and buccal mass. Fig. 7—The nerve ring in ventral view. B Mass, Buccal Mass. Bucc G, Buccal Ganglion. Ce G, Cerebral Ganglion. Cut Comm, Cu cerebral and pedal commissures. Ext Pro, External wall of the proboscis. J, Jaw C Bucc C, Left cerebro-buccal connective. L Pall N, Left pallial nerve. L Ped G, Left Pedal ganglion. L Pl G, Left pleural ganglion. L Pl 1 and L Pl 2, Left pleural nerves. L Pl-Pa C, Left pleuro-parietal connective. Oes, Oesophagus. Oes Art, Oesophageal artery. Or D, Invaginated oral disc. Or D N 1,2, Nerves to the oral disc. Oto. Otocyst. Ped A, Pedal artery Ped. N Lat. Small lateral nerves from the pedal ganglion. Pl N. B. W, Nerves from the pleural ganglion to the body wall. Pro A, Proboscideal artery. Pro N 1 2, Nerves to the wall of the proboscis. R Pall N, Right pallial nerve. R Pa-Visc C, Right parieto-visceral connective. R Pd G, Right pedal ganglion. R Pl Ped C, Right pleuropedal connective R Pl G, Right pleural ganglion Sal. G, Salivary gland Sb G. Subintestinal ganglion. Tent A. Tentacular artery. Zyg. Zygoneury.

is in close contact behind with the antero-dorsal surface of the pleural ganglion;, the cerebropleural connective in Struthiolaria, as in most Monotocardia, is so short as hardly to be separately recognisable. This short cerebropleural connective, the cerebropedal connective and the longer, horizontal pleuropedal connective (Text-fig. 7, Fig. 7, Pl Ped C) enclose on either side of the nerve ring a small 3-sided fenestra referred to by Bouvier as the “triangle laterale”. Each cerebral ganglion gives rise to a set of buccal and proboscideal nerves, together with the following four pairs of cephalic sensory nerves:— (i) Cephalic tentacular nerves, arising antero-laterally from the free face of the ganglion and running outwards to the tentacles (Text-fig. 6, C Tent N). (ii) A pair of much more slender Optic nerves (Op N), running to the optic peduncles. (iii) Cephalic tegumentary nerves, a pair of trunks (C Teg N) of equal thickness to the tentacular nerves, arising further back and supplying the integument of the head. (iv) Otocystic Nerves (Oto N), extremely slender trunks arising, one each side, from the ventral surfaces of the cerebral ganglia. The pedal ganglia (Ped G) are ovoid bodies, 1.5 mm in length, united by an extremely short pedal commissure. They form the most anterior part of the nerve ring, and give rise in front to two pairs of stout pedal nerves, of which those nearer the mid-line pass beneath the pedal arteries and proceed forward without branching for a good distance along the wall of the pedal sinus. The more lateral pair (Ped N Lat) follow a deeper course and commence to branch and rebranch on reaching the side walls of the foot. In addition, four smaller pairs of pedal nerves arise further back from the anterolateral edge of each ganglion. They pass directly to the body wall at the base of the foot. The pleural ganglia (Text-Fig. 7, Fig. 6, R Pl G, Fig. 7, L Pl G) are roughly spherical in shape, and are much smaller than the pedals They are widely separated from each other; a pleural commissure is never present in Gastropoda and the ventral moiety of the nerve ring is thus, in its primitive condition, open posteriorly. In Struthiolaria it becomes closed by the short right pleuroparietal connective, by which the subintestinal ganglion (Sb G) is drawn into the nerve ring, and by the zygoneury on the right side (Zyg) which secondarily connects the subintestinal and the right pleural ganglia. The pleural ganglia give rise to pleuroparietal connectives and as well to several important pleural nerves supplying the body wall in the trunk region. From the left pleural ganglion springs also the large left pallial nerve (L Pall N) passing obliquely backwards to supply the left side of the mantle, which—with the exception of the ctenidium and osphradium—is entirely innervated from this source. A much smaller nerve (L Pl I) supplies the musculature of the body wall on the left side, while a third pleural nerve (L Pl 2) arises mesially to the pallial nerve and runs back for some distance along the floor of the haemocoele before entering the musculature. From its dorsal surface, the right pleural ganglion gives off the prominent left pleuroparietal connective (Text-fig. 6, L Pl-Pa C), which runs to the supraintestinal ganglion, and forms the anterior section of the visceral loop on the left side. The right pleural ganglion has only one important nerve to the body wall, which corresponds in position with the left pallial nerve. This ganglion is linked with the subintestinal by the (right) zygoneury, a short curved con-

nective passing directly beneath the oesophagus. It should be noted that in Struthiolaria, as well as in Buccinum (Dakin, 1912) and other prosobranchs in which the subintestinal ganglion has moved forward, the right side of the mantle is innervated not from the right pleural ganglion, but by a right pallial nerve arising from the subintestinal ganglion and corresponding to the ctenidial nerve of the left side. In addition to the above nerves, the pleural ganglia also supply part of the wall of the trunk by a pair of nerves arising not from the ganglia themselves but from the pleuropedal connectives about half-way along. These nerves branch within the haemocoele, sending one branch directly to the body wall, the other backwards for some distance along the roof of the haemocoele. The Nerves of the Buccal Mass and Proboscis (Text-fig. 7, Fig. 6). Two systems of nerves supply the organs lying within the proboscis, the first derived from the buccal ganglia; the second, to the oral disc and the wall of the proboscis, proceeding directly from the cerebral ganglia. Along the anterior edge of each cerebral ganglion arises a series of six nerve trunks, which pass forward through the haemocoele of the proboscis, lying close together on either side of the oesophagus. Beginning near the midline, the first and second nerves in this series are respectively the dorsal and ventral oral tegumentary nerves, (Text-fig. 7, Fig. 6, Or D N 1, 2) which run close together as far as the buccal bulb, diverging there to break up into long, fine branches over the integument of the oral disc. The third nerve is distributed anteriorly on the side wall of the proboscis. The fourth is the stout cerebrobuccal connective (C Bucc C), which runs first alongside and finally below the oesophagus, taking a rather sinuous course forward to the buccal ganglion. At the side of this connective arises two tegumentary nerves of the proboscis (Pro N 1, 2), which break up at various levels along the proboscis to supply its muscular wall. The buccal ganglia (Bucc G) are small, spherical bodies located at the sides of the radular caecum, and united immediately behind the radula by a stout, transverse buccal commissure. From the anterior surface of each ganglion, four nerves run to the wall of the buccal bulb. The two nearest the mid-line are the ventral pharyngeal nerves; the third, the lateral pharyngeal nerve, branches over the sides of the buccal mass, while the dorsal pharyngeal nerve runs on to the roof and is usually concealed by the overlying salivary gland. Near the junction of the buccal commissure an odontophoral nerve runs inwards, at the side of the radular caecum, near the base of the odontophore. From the posterior face of each buccal ganglion a second series of nerves arises; these pass outward to the wall of the proboscis, where they break up into fine branches and mingle with the ultimate branches from the cerebral nerve supply. On the lower surface of the anterior part of the oesophagus, there are two pairs of extremely slender ventral oesophageal nerves, following a direct course backwards along the gut. The outer pair seem to spring from the cerebrobuccal connectives just before the junction of these with the buccal ganglia; but their origin is no doubt in the ganglia themselves. The inner pair are from the cerebral ganglia and appear to correspond with nerves from the original stomatogastric ganglia, which in Struthiolaria and other Monotocardia are quite suppressed as separate centres. The Left Pallial Nervous System. The pallial innervation is asymmetrical and on the left side is derived from two sources, the left pallial nerve and the ctenidial nerve. The pallial nerve breaks into two chief branches supplying the dorsal and the ventral portions of the pallial skirt respectively. The dorsal branch,

running with the pallial vein and artery, gives off a rather large branch to the siphonal lappet, and continues around the mantle, deep to the osphradium, giving off frequent branches to the pallial margin. At the mid-dorsal line the left pallial nerve breaks into a series of fine branches which ultimately anastomose with similar branches of the right pallial trunk. There is a similar anastomosis in the ventral mid-line of the pallial skirt. The ctenidial nerve (Text-fig. 6, Ct N) is a slender trunk running from the supraintestinal ganglion, and running out parallel to and behind the left pallial nerve. It crosses below the osphradium at the point where that organ diverges at an angle from the gill. Here it gives off a small osphradial nerve which joins the axis of the overlying osphradial ganglion, a long, slender cord running the whole length of the osphradium. A short distance beyond the wall of the trunk, the left pallial and the ctenidial nerves are placed in communication with each other by a short cross connective, the left dialyneury (Text-fig. 6, Fig. 5, L Dial). Such a connection forms, in the prosobranch nervous system, a peripheral link between nerves derived from the left pleural and the topographically left parietal ganglion. As a result of chiastoneury, the left pallial and the ctenidial nerves are centrally connected with opposite pleural members. The pleural ganglia themselves have no direct connection with each other—hence the functional importance of a peripheral connection by the dialyneury on the left, and by the analogous zygoneury on the right. The ctenidial nerve gives rise posteriorly to numerous slender branches, which run outwards obliquely, cross beneath the osphradium, and finally—just before reaching the axis of the gill, break up into very numerous, parallel filamentar nerves (Text-Fig. 6, Fig. 5, Fil N) running along the individual gill filaments of the gill. As they run towards the gill these filamentar nerves branch and anastomose with each other in the mantle wall. The Visceral Loop. The forward migration of the sub-intestinal ganglion has obscured the figure-of-eight disposition of the visceral loop in Struthiolaria. The right (or inferior) pleuroparietal connective is virtually eliminated and the point of chiastoneury thus shifts forward to lie within the nerve ring itself. The right half of the visceral loop now consists essentially of a long right parietovisceral connective and the left half incorporates both the left pleuroparietal connective (L Pl-Pa C) and the left parietovisceral connective (Text-fig. 6, Fig. 5, L Pa-Visc C). The left parietal or supraintestinal ganglion (Text-fig. 6, Fig. 5, S Int G) is a large, triangular body, located just inside the wall of the left side of the trunk at the point of departure of the pallial artery. In addition to the ctenidial nerve, it has three much smaller branches running backwards in the body wall. The left parietovisceral connective is much more slender than the pleuroparietal and passes backwards along the left compartment of the cephalopedal venous sinus. It presently buries itself in the muscular floor of the haemocoele, through which it continues back to the left visceral ganglion. The right parietovisceral connective (Text-Fig. 7, Fig. 7, R Pa-Visc C) is an equally slender trunk, lying in the right compartment of the cephalopedal sinus and, further back, in the superficial muscles of the floor of the haemocoele. There are two visceral or “abdominal” ganglia, lying at the extreme posterior end of the trunk on the broad upper surface of the columellar muscle, within

the subrenal blood sinus. The right visceral ganglion (Text-fig. 6, Fig. 5, R Visc G) is considerably the larger, lying just mesially to the albumen gland in the female and alongside the renal vas deferens in the male. This ganglion has a very characteristic shape; it consists of a pale yellowish body, to which is attached an elongate whitish lobe which replaces the transverse visceral commissure for about one-third of the distance between the two ganglia. The left visceral ganglion is much smaller, lying on the extreme left side, between the oesophagus and the cephalic aorta. The nerves from these ganglia supply directly the whole of the visceral mass, including the stomach, the digestive gland, the renal organ, the heart and the pericardium as well as the posterior part of the intestine and the genital ducts. The long visceral nerve (Text-fig. 6, Fig. 5, Visc N) arises posteriorly from the right visceral ganglion and runs back over the concave surface of the digestive gland to the tip of the spire. To the right, the same ganglion gives off a stout rectal nerve (Text-fig. 6, Fig. 5, Rect N), as well as two much finer nerves to the genital duct. Towards the mid-line arises a renal nerve, breaking up over the floor of the renal sac. The elongate lobe of this ganglion gives off two slender nerves towards the middle line, supplying the integument nearby. From the left visceral ganglion a small nerve branches over the ventral surface of the cephalic aorta, while two larger nerves pass backwards, the first following the aorta back to the ventricle and the second passing back over the oesophagus to its final distribution upon the wall of the stomach. Comparative As compared with the Struthiolariidae, the Aporrhaidae—their closest relatives—show very few advanced features in the nervous system. As described and figured by Bouvier, and personally checked by the writer in Aporrhais pes-pelicani, the cerebropleural and cerebropedal connectives are unusually long in this family, as are also the pleuropedals. The pedal ganglia lie much more posteriorly, having not yet attained the forward position they occupy in Struthiolaria. The subintestinal ganglion retains its primitive position, remote from the nerve ring, the right pleuroparietal connective being of equal length with the left and the right zygoneury correspondingly long. The third family of the Superfamily Strombacea is the Strombidae, and in many respects—particularly in external form and mode of locomotion—these snails are much more highly specialised than either the struthiolariids or the aporrhaids. In the nervous system, however, they more nearly resemble the primitive family, the Aporrhaidae. By reference to earlier figures for Strombus gigas (see Simroth (1907), as checked by the writer's dissections of a species of Conomurex, it can be seen that the pedal ganglia retain their aporrhaid shape. The cerebropedal and pleuropedal commissures remain very long. The cerebral and pleural ganglia are in close contact, as in Struthiolaria, and in this feature a primitive character of the aporrhaids has been lost. The subintestinal ganglion is not incorporated in the nerve ring, but nevertheless shows a greater tendency to shift forward than in the Aporrhaidae, the right pleuroparietal connective being only about half as long as the left. In the visceral ganglia, all the Strombacea so far examined seem to be strikingly alike, and this resemblance extends to the details of the innervation of the gill, osphradium and left half of the mantle. In general such trends as the approximation of ganglia—either the two members of a pair, or members of different pairs on the same side—and the shortening

of commissures and connectives have been held to be reliable in indicating evolution from a “primitive” towards an “advanced” grade within a particular family or superfamily. In discussing the evolution of the Strombacea, the present writer (1950, 1951) has already considered that the family Aporrhaidae represents the primitive stock, and that the Strombidae and the Struthiolariidae are families advanced in various directions upon a basic aporrhaid pattern. The Struthiolariidae appear superficially the closer to the Aporrhaidae; they have avoided all of the more bizarre specialisations shown by the Strombidae, and may be said to have carefully observed the principle of “minimal adaptive specialisation”. Yet in many features of their mode of life, and in particular the acquisition of ciliary feeding, they have reached a higher level of organisation than either of the other two families. The present results on the nervous system have an added interest in giving emphasis to the distinctness of the struthiolariids from the aporrhaids or strombids, and in pointing clearly to the advanced position occupied by this family among the Strombacea. As a measure of the relationships between the members of larger groups than superfamilies, the nervous system is likely to have a restricted value. For example, many of the more advanced families of prosobranchs have acquired a “concentrated” nerve ring by the incorporation of one or both of the parietal ganglia. In many cases this character must have been separately developed along parallel lines, and to try to incorporate in a single group all prosobranchs with a concentrated nerve ring is likely to bring about very misleading conclusions. Haller (see Simroth (1907)) attempted this in his recognition of “longicommissurate” and “brevicommissurate” Taenioglossa, with the result that the Aporrhaidae and the Strombidae are found in the first group, and their closest relatives, the Struthiolariidae, removed from them into the second. Literature Cited 1. Crofts, Doris R., 1929. “Haliotis,” L.M.B.C. Monograph, 29, 174 pp., S. Pl., Liverpool. 2. Fretter, Vera., 1941. The Genital Ducts of Some British Stenoglossan Prosobranchs. J. Mar. Biol. Assoc. U. K., 25. 173-211. 3. —— 1943. Studies in the Functional Morphology and Embryology of Onchidelli celtica (Forbes and Hanley) and their bearing on its Relationships. Ibid, 25. 685-720. 4. Dakin, W. J., 1912. “Buccinum,” L. M. B. C. Monograph. 20, 123 pp. S. Pl., Liverpool. 5. Moore, J. E. S., 1899. The Mollusca of the Great African Lakes. III. Tanganyikia rufofilosa and the genus Spekia. Q. Journ. Micr. Sci. 42, 155-185. 6. Morton, J. E., 1950. The Struthiolariidae: Reproduction, Life History and Relationships. Trans. Roy. Soc. N. Z., 78, 451-463. 7. —— 1951. The Ecology and Digestive System of the Struthiolariidae (Mesogastropoda). Q. Journ. Micr. Sci. 92, 1-25. 8. Nisbet, R. H., 1951. Nervous Control of Feeding Movements in a Trochid (Osilinis lineatus). Contrib. to Section D, Brit. Ass. Adv. Sci., 1951. 9. Pelseneer P., 1906. Mollusca. A Treatise on Zoology (ed. E. Ray Lankester) Pt. 5 London. 10. Simroth, H., 1907. Mollusca in Dr. H. G. Bronn's Klassen und Ordnungen des Tierreichs III 2, Gastropoda Prosobranchia. 11. Yonge, C. M., 1923. Studies on the comparative physiology of digestion. I. The mechanism of feeding, digestion and assinulation in the lamellibranch Mya. Brit. Journ. Exp. Biol. 1 15 Dr. J. E. Morton Dept. of Zoology Queen Mary College University of London Mile End Road, London. E.1.