Art. XXXIX.—Vascular System of Siphonaria obliquata Sowerby.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 44, 1911, Page 374

Art. XXXIX.—Vascular System of Siphonaria obliquata Sowerby. By A. J. Cottrell, M.A., M.Sc. (N.Z.). Communicated by Professor Benham. [Read before the Otago Institute, 3rd October, 1911.] In a previous paper (Trans. N.Z. Inst., vol. 43, 1911, p. 582) I described the general anatomy of Siphonaria obliquata, the common limpet-like marine Gastropod, whose affinities are still undecided; for, while some authorities incline to the view that it is a modified Opisthobranch, others, and perhaps the majority, regard it as a Pulmonate. I now supplement that article by giving here an account of the vascular system, which was omitted purposely from that paper. The adaptation of Siphonaria to a life along the sea-shore, where at times it is below water, at other times above, is well illustrated by its “dipnoan” character, having, that is, both a lung and a gill, each with its own independent blood-supply. If, as seems the more probable, it is a Pulmonate which has left its terrestrial home and ventured back into the original habitat of the Gastropod molluscs, we must look on the gill as a new structure, analogous to those of such forms as the Prosobranch, Patella (limpets), and the Opisthobranch, Pleurophyllidia, which has arisen in relation to this new mode of life—that is, it is an “adaptive” gill, as in these forms. Its structure is not that of a typical ctenidium, as I pointed out in my previous article, though it has some resemblance to certain gills amongst the Opisthobranchs. We already know a few Pulmonates which have taken to an aquatic life in which a gill is present. Thus, in Isidora, Protancylus, and others there is a single-folded lamina; but in Siphonaria, as I have described it in my previous article (p. 585), there is a series of independent laminae arranged in a semicircle round the mantle-cavity, each lamina bearing secondary laminae, so that it is much more complex than the gill in the above Pulmonates. Lang regards these pulmonate gills as probably ctenidia, though Pelseneer and others hold them to be “adaptive” gills. On the other hand, supposing that Siphonaria is an Opisthobranch which has become able to remain out of water for a considerable portion of each day, and to breathe air during this period, we must suppose that it is the lung that is the new thing; and although, so fan as I am aware, no Opisthobranch has been found to have a lung, yet amongst the Prosobranchs several genera, belonging to different families, have developed a network of bloodvessels on the mantle-roof; so that the cavity acts as a lung, and the genera can be arranged in series, in which the ctenidium gradually decreases in size and importance till it remains as a mere vestige (as in Cerithidea). Hence, whatever view is taken as to the affinities of Siphonaria, it remains an extremely interesting form to biologists, having either an adaptive lung or an adaptive gill. The Heart. The heart lies in the pericardium on the left side of the dorsal surface of the body, just in front of the middle of the length. The position of the heart, as seen in the animal after the shell has been removed, is shown in fig. 7 of plate 28 of my former article. The walls of the pericardium

are formed by a thin tough transparent membrane. The heart, as usual, is two-chambered, the ventricle being situated to the left of the auricle, its apex pointing to the left and downwards, while the auricle lies almost horizontally and transversely to the body. The walls of the auricle are thin, transparent, and extremely delicate. When this chamber is distended with blood the bluish tinge of the latter may be noticed, Fig. I. Dissection of the animal, showing the chief arteries; × 2. Dorsal wall of body (mantle) with heart and pericardium turned to the left, exposing the ventral surface of the heart. a., anus; ao., aorta; b.a., superior buccal artery; c.a., cephalic aorta; d.g., digestive gland; f., edge of foot; f.b.c., floor of body-cavity and upper surface of foot; gn.a., genital artery; g.a., gastric artery; g.b.a., gastro-buccal artery; h.g., hermaphrodite gland; i., intestine; p.a., penial artery; p., penis; pro., prostate; v.a., visceral aorta; v., ventricle; sth., spermotheca; st., stomach r., rectum; w.a., artery to body-wall; m.t., mantle turned aside. due to the presence of the respiratory pigment, haemocyanin, characteristic of the respiratory fluid of Mollusca and Arthropoda. In this distended condition the auricle is of a more or less cylindrical shape, the long axis being transverse to the body. Two large veins discharge blood into this chamber, uniting just as they reach it. From the auricle the blood or haemolymph passes between the two auriculo-ventricular valves into the ventricle. Owing to the fact that it possesses thicker walls the

ventricle is of more definite and constant shape than the auricle; from above it is broad at the auricular end and tapers to the other extremity, where it ends in a blunt point. After removal of the auricle the valves can be seen partly open, leaving the silt-like orifice through which the blood passes to the ventricle. When the ventricle contracts, the lips of these valves come together and close the passage. This action can be well seen by filling the chamber with water and gently squeezing it. The walls of the ventricle are stout and muscular, and from the exterior can be seen the multitude of muscular fibres running in varying directions in them. Most of the fibres converge on a stout ring formed round the exit of the aorta; their other ends, often branched, are attached to the walls. Arteries. The aorta leaves the ventricle near the middle of its ventral side, and, piercing the pericardial wall, to which it is firmly attached, takes a course almost directly transversely towards the right and a little backwards, passing between the anterior lobes of the digestive gland till it reaches the intestine. At this point it bifurcates, giving rise to the cephalic aorta (fig. 1, c.a.), supplying the anterior region of the body, and the visceral aorta (v.a.), supplying the posterior organs. This passes backwards among the lobes of the digestive gland, and divides into three main branches, which carry blood to that organ, to the hermaphrodite gland, intestine, and posterior end of the stomach. For the purpose of studying the distribution of the arteries I injected coloured “starch injection” into the ventricle, and was successful in obtaining a beautiful preparation showing even the smaller branches, but could not afford time, nor did I consider it worth while, to work out the distribution of the visceral aorta in further detail. The cephalic aorta passes to the right over the dorsal surface of the viscera, and comes in contact with the body-wall (i.e., the floor of the pallial chamber) just below the kidney. Continuing its course to the right, it curves forwards till it reaches the spermothecal duct, in front of which it passes downwards and forwards on to the floor of the body-cavity. Just before reaching this point it gives off the genital artery (gn.a.), which, passing backwards, supported by a thin membrane, gives rise to the vessels of the spermotheca and genital duct. The cephalic aorta now runs forwards for a short distance on the floor of the body-cavity, and after giving off a vessel (w.a.) which disappears into the muscles of the right body-wall and foot it turns to the left and reaches the nerve-collar, through which it passes below the oesophagus. Fig. 2. Arties on the ventral side of stomach; × 4. b.a., superior buccal artery; g.b.a., gastro-buccal artery; g.a., gastric artery; oe., oesophagus; st., stomach.

A little before reaching this point the cephalic aorta gives rise to a large branch—the gastro-buccal artery (g.b.a.), which passes to the left along the floor of the body-cavity, reaching the stomach on the ventral surface a little behind the point where the oesophagus (fig. 2) enters. Here it an once bifurcates, one branch—the superior buccal artery (b.a.)—going forwards to the buccal mass, and the other—the gastric artery (g.a.)—backwards along the ventral wall of the stomach. The superior buccal artery passes forwards as a large vessel along the left latero-ventral side of the oesophagus, and turns up with the latter on to the dorsal surface of the buccal mass, giving off small vessels to the oesophagus on the way. On reaching a point a little behind the buccal ganglia it bifurcates into right and left branches, A and B respectively (fig. 3). Branch A passes under the oesophagus to the right side, and gives off the artery which supplies the salivary gland. It then passes on to the latero-dorsal surface of the oesophagus and sends out a branch to the right buccal ganglion. These ganglia are well supplied with blood-vessels, and when the latter are well injected appear to be encased in an envelope of small arteries. An artery also runs on to the commissure connecting these ganglia. Beyond this point branch A passes to the anterior end of oesophagus, and finally dips into the anterior muscles of the buccal mass. Branch B of the superior buccal artery has a similar distribution on the left side. Fig. 3. Arteries supplying the dorsal surface of buccal mass; × about 4. A, right branch of buccal artery, which passes below the oesophagus; B, left branch of buccal artery; b.g., buccal ganglion; b.a., superior buccal artery; r.s., end of radular sac; s.a., right salivary artery; s., salivary gland; oe., oesophagus. Thus this branch of the gastro–buccal artery, which I have called the superior buccal, to distinguish it from another vessel (the inferior buccal artery), supplies the oesophagus, salivary glands, buccal ganglia, and some of the dorsal muscles of the buccal mass. Fig. 4. Arteries on the floor of the body supplying the head and foot, as seen when the buccal mass is severed; × 2. A, B, and C, the three main arteries supplying the head; B is the inferior buccal artery, and is cut short; c.a., cephalic aorta (which is much swollen at the point where it branches as it passes between the pedal ganglia); g.b.a., gastro-buccal artery; m., mouth (buccal mass being removed); p.a., penial artery; pd.a., pedal artery; pd.g., pedal ganglion. The gastric artery (fig. 2) takes a backward course on the ventral side of the stomach, giving off branches to the ventral surface of the stomach; the extreme posterior of this organ, however, is supplied by a branch of the visceral aorta.

Returning to the cephalic aorta, which we traced above as far as the nerve-collar, we find that as it passes between the pedal ganglia it breaks up into a number of large vessels, some of which run forwards on the floor of the body-cavity, supplying the organs and walls of this region, and some pass backwards, supplying the foot (fig. 4). Running forwards we have three main arteries—A, B, C (fig. 4). The firs of these, A, supplies the ventral wall of the head, penis, and muscles round the mouth. It would appear from Captain Hutton's figure that the penial artery was mistaken by him for the vas deferens.* Trans. N.Z. Inst., vol. 15, pl. 17, fig. B. c. B, which I term the inferior buccal artery, arises to the left of A, goes directly into the buccal mass on its ventral side, and supplies almost the whole of this organ. To the left of B again is the branch C, which is the largest of the three: very near its origin it bifurcates into right and left branches, the latter corresponding to A of the right side. The right branch runs forwards for a short distance, where it bifurcates, the rami entering the muscular ventral wall of the head. Running backwards from the cephalic aorta at this point are two large arteries, right and left, which disappear among the muscles of the foot (pd.a.). Veins. The distribution of the vems is illustrated in my previous article (Trans. N.Z. Inst., vol. 43, pl. 28, fig. 7, and pl. 29, fig. 2). In the foot and body-walls are numerous large blood-vessels or spaces, and into these, as far as I have been able to make out, most of the blood eventually makes its way. Part of this blood is collected by a large vein which runs vertically up in the left body-wall near the end of the gill; on reaching the dorsal surface it bifurcates, one branch —posterior renal vein—being distributed to the kidney, and the other—the afferent branchial vein—running round the posterior border of the gill, and distributing blood to the gill-lamellae. Near the respiratory orifice it gives off a large branch—the anterior renal vein—which runs between the gill-lamellae on to the kidney close to the renal papilla, a process of which surrounds the vessel between its origin and the kidney. This vessel has been figured lightly, as it lies deeper than the other vessels, and its reference-line has been misplaced in the figure (vol. 43, pl. 28, fig. 7). Connected with the afferent branchial vein along its whole length there are a large number of pallial vessels. The blood passes from this vein through the gill, where it is aerated, into the efferent branchial vein, which runs round the anterior margin of the gill. This vein returns blood to the auricle partly by a vein, leaving it half-way along the gill and crossing the kidney, from which it receives several small vessels, and partly by the efferent pulmonary vein, which it joins at the right end of the gill. This latter vessel receives blood from the efferent vessels of the lung and enters the auricle together with the efferent vessel crossing the kidney. Another large vein receiving blood from the body generally is the afferent pulmonary vein, which emerges from the body-wall just in front of the pericardium, and runs round the anterior margin of the lung, giving rise to the afferent vessels of the lung. The blood passes through these, and reaches the efferent vessels, which carry it to the efferent pulmonary

vessel, and so to the auricle. There is thus always arterial or aerated blood in the heart, and purification of the blood is effected in the mantle, which is everywhere very vascular, as well as in the two respiratory organs connected with it. For the purpose of tracing out the blood-vessels I injected from the heart backwards and forwards. From the auricle I injected backwards “berlin blue” very successfully into the vessels of the gills, lungs, and kidney; and forwards through the ventricle I tried Parker's “starch injection” and “glycerine carmine”: the former gave me the most satisfactory results.