Art. LI.—Anatomy of Siphonaria obliquata (Sowerby).

By A. J. Cottrell, M.A., M.Sc. (N.Z.).

Communicated by Professor Benham.

with the rock, though not so closely as does that of the limpet. This is the condition in which I have most frequently found them. The thick body-wall surrounding the internal organs on the sides (Plate XXVIII, fig. 1; bw) is composed of a dense mass of muscular tissue, the fibres of which run in all directions, thus rendering a great variety of movement possible.

In all regions of the body-wall forming the sides of the mollusc some of the cells of the epidermis are differentiated to form multicellular mucous glands, which project inwards among the muscles. The cavity of each is club-shaped, the narrow part ending externally in a pore. Around this cavity the cells are arranged—large glandular vacuolated cells, the outlines of which are not distinct. These cells secrete the mucus or material which forms it. It is stored in the cells in the form of granules, which stain very deeply in borax carmine. These can be seen even under the low power as conspicuous red masses. The secretion is discharged into the cavity, and on the contraction of the muscles in the vicinity it reaches the exterior. These glands are very numerous, and some of them are of considerable size. When the animal is disturbed it shrinks up, causing a considerable quantity of mucus to be exuded.

Foot. Plate XXVIII, figs. 2, 3.

The foot is of considerable size, consisting of the whole of the ventral surface behind the head. In the case of specimens preserved in formol the foot is much shrunken (Plate XXVIII, fig. 2; f); but I managed to kill a fine specimen in a very well-distended condition, and then hardened it in a strong spirit. This specimen shows very well the extent of the foot (Plate XXVIII, fig. 3; f). There is a slight propodium, on to which a large pedal gland opens in the median line. This gland extends through the whole length of the foot (Plate XXVIII, fig. 3; pg, p). The foot, like the body-wall, is very muscular, and has the same structure as the latter, except that here are no mucous glands, and in this region the epidermis is composed of columnar cells. The beautiful movement of the muscular foot may be well seen if a living specimen be made to creep up the inside of a glass vessel. The action is similar to that of a snail.

Head. Plate XXVIII, figs. 1, 2, 3.

The head is marked off from the body by a slight furrow running transversely across the ventral side immediately in front of the foot, and extending upwards for only a short distance on each side (Plate XXVIII, figs. 1 and 2). Anteriorly the head is convex, and the thick wall investing it is continuous with the body-wall described above, and has the same structure. Ventrally the head is flat, and is in contact with the surface to which the mollusc attaches itself. Like the foot, this part of the head is used for effecting locomotion, and has the same histological structure. When distended the right and left lobes of the head can be well seen (Plate XXVIII, fig. 3; rl, ll). No tentacles are present on the head, though part of these lobes on either side may correspond to tentacles.

External Orifices. Plate XXVIII, figs. 1, 2, 3; Plate XXIX, figs. 2, 3.

The mouth is situated in the middle of the ventral side of the head; it is a transverse slit, somewhat crescentic in outline, and median in position (Plate XXVIII, fig. 3; m).

On the right side of the body is the respiratory orifice, situated halfway along that side (Plate XXVIII, fig. 1; r). In large specimens it is about

Morphology and Histology.

The kidney is of a pale-brownish colour; it consists of two regions, dorsal and ventral, folded one over the other (Plate XXIX, fig. 2; dk, vk). The former is attached to the mantle, and near the respiratory orifice, close to the gill, has a small spherical swelling—the renal papilla—on which the renal pore opens (Plate XXIX, fig. 2; rp). The papilla is always more or less concealed by the gill. The ventral region is somewhat leaf-shaped in outline, and is attached in the same way to the floor of the pallial chamber. The whole of the kidney is glandular, and contains no extensive cavity as is seen in the kidney of, e.g., Anodonta or Unio. On the left side the kidney lies close to the pericardium, the right end of which it partially surrounds.

The gill is composed of a series of laminae separate from one another, and each independently connected to the mantle by one of its lateral edges, the other edge hanging freely into the pallial chamber (Plate XXIX, fig. 2; g). As noticed above, this series of gill-laminae forms a semicircle round the posterior end of the pallial chamber. Running round the gill on its posterior side is a large vein—the afferent branchial vein; and there is another having a corresponding position on the anterior side—the efferent branchial vein. With these vessels the ends of each lamina are connected, the posterior end with the former vessel, and the anterior with the latter. Each lamina tapers at each end to a small tubular structure carrying a branch from one of these vessels on to the lamina. Between these two extremes each lamina spreads out and is thrown into longitudinal folds, and on the edges or ridges formed by these folds secondary laminae often arise. Dorsally some of these folds may unite with the mantle, especially those laminae forming that part of the gill near the respiratory orifice, where the gill is most strongly developed and the folding is very complicated.

Each surface of a gill-lamina is covered by a layer of ciliated epithelium, consisting of a single layer of cells, which are somewhat cubical, and contain large nearly circular nuclei. These nuclei stain very deeply in borax carmine. Between these two surfaces lies connective tissue containing bloodspaces. These are no skeletal elements in connection with this gill.

Fig. 1.—Transverse Section of Gilllamina; × 50.
bv, Anterior renal vein going to the kidney; e, edge of lamina attached to mantle; e′, free edge of lamina; gl, gill-lamina; gl′, secondary gill-lamina; m, mantle forming roof of pallial chamber.

The blood flows from the posterior vessel into the gill-laminae, and, passing through the blood-spaces in them, is brought close to the water in the pallial chamber, and is thus aerated.

The gill is ciliated, and the water surrounding it is kept in motion by this means. The blood passes on, and is gradually collected into larger spaces, and finally, reaching the efferent vessel, is carried to the heart.

This gill seems to be quite different from the typical ctenidium; it does, however, resemble certain parts of the ctenidia of some opisthobranchs.

Alimentary System. Plate XXVIII, fig. 6; Plate XXIX, fig. 3.

The mouth perforates the thick muscular wall of the ventral side of the head in a direction vertically upwards, and leads into the cavity of the buccal mass—the “buccal cavity.” The buccal mass is situated in the head, the cavity of which it practically fills. It is spherical in shape, of a dark colour, and is very muscular. Lying on the dorsal surface of the buccal mass, and usually almost concealing it, is a pair of salivary glands, one on each side. The oesophagus passes away from the buccal mass on the dorsal surface, arising near the anterior end, and, following the curve of the buccal mass, it passes backwards and downwards till it reaches the floor of the body-cavity, where it soon enters the stomach, which it joins on the ventral region of the anterior surface, the transition from oesophagus to stomach being very marked (Plate XXIX, fig. 3; oe, st). The stomach extends from the buccal mass to the posterior end of the creature, occupying a large part of the left half of the body-cavity. When distended it is cylindrical, and is of smaller diameter in the middle than at the two ends (Plate XXVIII, fig. 6; st). The digestive gland is very extensive; it occupies a large part of the posterior half of the body-cavity, and its lobes completely surround the posterior half of the stomach. It is really divisible into a right and a left part; the left is the larger, and the digestive juice is collected from its various lobes by ductules into the main ducts, which enter the stomach close together on the left side near the posterior end. The right part is much smaller; its duct joins the stomach on-the right side, also near the posterior end (Plate XXVIII, fig. 6; dg, dg′).

The intestine passes away from the left side of the stomach, at the hinder end, posterior to and below the ducts of the digestive gland (Plate XXVIII, fig. 6; i). It is at first wide, but soon becomes a narrow thick-walled tube. After passing forwards and upwards it coils twice among the lobes of the digestive gland, and comes on to the dorsal surface on the right side, near the posterior end of the creature. It then runs forwards as the thin wall rectum, which on reaching about the middle of the length of the animal takes a sharp turn to the right, ending at the anus on the lower side of the respiratory orifice. Just inside the mouth, at the entrance to the cavity of the buccal mass, there is a chitinous jaw. This jaw is crescentic in outline, and extends from side to side of the buccal cavity on the anterior side of the mouth. When the creature is fully distended the jaw comes to the surface, and assists in breaking up food. It is attached at the lower end of the thick muscular anterior wall of the buccal mass to the chitinous lining of this region. The structure of this jaw is very similar to that of a radula; it is composed of a basement membrane of chitinous material, beset with numerous smal teeth. These teeth are cylindrical, pointed, all alike, and very numerous.

Captain Hutton seems to have incorrectly described this jaw as “Anterior margin papillate—rest smooth.”^*

The epithelium lining the buccal cavity is everywhere covered by a layer of chitin, which varies in thickness.

This cavity is blocked up by two similar and symmetrical muscular masses, one on each side of the middle line. Each is simply a muscular thickening of the wall of its respective side. Also projecting into the buccal cavity, behind these structures just described, is the “odontophore,” with the radula. The radular teeth are comparatively small, very numerous, as in Helix, and are carried on a chitinous basal membrane. They are

This string is simply a continuation of the glandular tissue; it passes over the oesophagus, under the cerebral commissure—i.e., it passes through the nerve-collar. The secretion of each gland is collected by ducts which ramify through the gland into a main duct. Each of these ducts (right and left) leaves its respective gland near the buccal ganglion, and, passing into the buccal mass close to the cerebro-buccal connective, discharges into the buccal cavity near the exit of the oesophagus. The stomach is quite different from the corresponding region in Helix: there is no crop as in Helix, and whereas the stomach of Helix is very small, that of Siphonaria is very large. But it may be urged that the anterior part of this region which I have called stomach may correspond to the crop of Helix, especially as the diameter of the tube is slightly less in the middle than at the ends. The posterior end of the stomach, as I have used the term, would then correspond to that of Helix. Yet for the following reasons I prefer to call it all the stomach: (1) The transition externally from oesophagus to stomach is very marked (Plate XXVIII, fig. 6; st), while it is not well marked in Helix; (2) the digestive fluid is discharged into the posterior end of one large chamber and is mixed with the food in that chamber, and this does not take place in Helix, but the food passes out of the crop to the small stomach further on, into which the digestive glands open.

Nervous System. Fig. 5, and Plate XXVIII, fig. 6.

The nervous system is characterized by the great concentration of its ganglia in the anterior part of the body. It consists of a ring of nervetissue surrounding the oesophagus immediately behind the buccal mass at the point where the oesophagus reaches the floor of the body-cavity (fig. 5; Plate XXVIII, fig. 6, n). The ganglia are confined to the lateral and ventral portions of the ring. In life they are of an orange colour, and can thus be readily recognized.

There is a pair of cerebral ganglia situated one on each side of the laterodorsal surface of the oesophagus, lying almost on the sides. These are put into connection by a stout commissure which passes over the oesophagus (fig. 5; cc). Each cerebral ganglion gives rise to six or seven nerves, which innervate the cephalic region. Two of these nerves on each side go to the region round the mouth, and the others are distributed to the muscles and sense-organs of the floor and walls of this region, including the eyes.

From each of these ganglia a connective goes through the muscles of the buccal mass to the buccal ganglia. The buccal ganglia lie close together on the dorsal surface of the buccal mass under the oesophagus, at a point immediately behind where the latter becomes free of the buccal mass (fig. 5; b). These ganglia are ovoid in shape, and are connected by a well-defined commissure. They give off nerves supplying the muscles of the buccal mass.

The pleural ganglia (fig. 5; pl) lie very close to the cerebral, and practically abut upon them, so that the cerebro-pleural commissure is quite short. The left pleural ganglion gives rise to a connective passing straight across to the visceral ganglion, which lies very near to the right pleural ganglion, being separated from the latter by a very small ganglion, probably the representative of the parietal. Thus the right viscero-pleural connective is very short, and consequently the whole visceral loop is likewise very short, and the number of ganglia on it is reduced (fig. 5; pv, pa, v). Each pleural ganglion also gives off several nerves. From the left a large nerve

runs obliquely across the floor of the body-cavity, and is distributed to the walls and the mantle. There is a corresponding nerve from the right pleural, but it does, not run so far back, going on to the body-wall just behind the penis. The pleural ganglion on this side also gives a stout nerve to the penis.

The visceral ganglion gives off a nerve passing back to the stomach (fig. 5; g), and another obliquely to the right; the branches of this latter nerve go to the body-wall and rest of the viscera. Close to the pleural ganglia on the underside of the oesophagus lie the pedal ganglia, one on each side. These are connected by a stout commissure which is much shorter than the cerebral commissure. It is supplemented by a second (para-pedal) commissure of about half its own thickness (fig. 5; p, pc, pc′). The pedal ganglia are, of course, connected to the cerebral by connectives. These lie anteriorly, and so close to the other ganglia that they are not easily seen. The pedal ganglia give rise to several stout nerves, which innervate the muscles of the foot.

Fig. 5.—Nervous System.
b, Buccal ganglion; be, cerebro-buccal connective; c, cerebral commissure; el, external labial nerve; g, gastric nerve; il, internal labial nerve; pa, parietal ganglion; pc, pedal commissure; pc′, para-pedal commissure; pl, pleural ganglion; pl.n, pleural nerve; pn, pedal nerves; pn′, penial nerve; p, pedal ganglion; pv, pleuro-visceral connective; v, visceral ganglion.

Sense-organs. Plate XXVIII, fig. 4.

Distributed all over the external surface of the body are tactile organs, as evidenced by the fact that when a part of the epidermis is stimulated the creature withdraws that region, and so adjusts itself to its environment.

I looked for an otocyst, but was unable to find one.

A pair of eyes is present. Each is situated on the anterior region of the head, well to the side, and symmetrically placed with regard to the other (Plate XXVIII, 4; e). I discovered them by cutting a series of sections through the head of a young specimen. Each appears to lie imbedded in the muscles of the cephalic wall, at the base of a little depression which deepens when the animal shrinks up, and the eye is concealed. Usually the eyes cannot be distinguished in an external examination, but two specimens I managed to get so well distended that the eyes were clearly visible. Captain Hutton, in his brief account of the New Zealand Siphonaridae, says no eyes are present.^*

The eye is of the usual molluscan type—densely pigmented, and containing the usual homogenous lens filling its cavity.

Fig. 6.—Reproductory System; × 2.
cd, Common duct—glandular region; cd′, common duct—non-glandular region; hd, hermaphrodite duct; hd′, small process formed of coil of hermaphrodite duct; hg, hermaphrodite gland; hg′, point at which hermaphrodite duct enters the common duct; p, penis; pro, prostate; sthd, spermotheca duct; sth, spermotheca; sv, seminal vesicle; gp, genital pore.

Reproductive System. Figs. 6, 7, and Plate XXIX, fig. 3.

Siphonaria obliquata is hermaphrodite; the organs of reproduction are both large and complicated, and comprise a hermaphrodite gland, a large glandular common duct, spermotheca, and a large penis (fig. 6; hg, cd, sth, p). These occupy a large part of the right half of the body-cavity. The genital products arise in the hermaphrodite gland, a dull orange-coloured organ lying partially imbedded in the digestive gland on the right side, near the posterior end of the body. It consists of several lobes, and on gently separating these there can be seen issuing from them little white ducts, which unite to form a main duct—the hermaphrodite duct. This

Notes on preserving, etc.

For preserving my specimens for dissection I used both formol and alcohol. The former has the advantage of preserving the natural colours, and this fact is especially useful in studying the nerve-collar, the ganglionic masses being orange-coloured. However, formol renders the tissues, with one exception, very hard; this exception is the glandular region of the common duct. In formol specimens I found this to swell up and become very brittle, so as to make it almost impossible to dissect. This organ preserves better in alcohol, and my drawings are from spirit specimens.

I tried several methods of killing Siphonaria so as to get the specimen well distended. I found this end was accomplished best by leaving freshly collected specimens for four or five days in a dish with a little sea-water in the bottom, then getting them gradually into fresh water, and thence into spirit or formol.

For fixing the tissues for histological purposes I used corrosive sublimate, corrosive sublimate plus 5 per cent. acetic acid, and glacial acetic acid. With all I obtained good results, but where I was able to make a fair comparison between them I found that in the majority of my preparations the last reagent gave me the best results.

For staining pieces of the tissues to be mounted whole I used alum carmine; for sections borax carmine and picro-carmine gave good results, except in the case of the genital duct, which I found difficult to get well stained: this organ requires three to four days' soaking in the stain. This difficulty arises in all probability owing to its size. In some preparations I used picro-nigrosine to show up the connective tissue, which it stains blue.

Affinities of Siphonaria obliquata.

Explanation of Plates XXVIII, XXIX.