Alimentary System. (Fig. 3.)
The mouth (m) is placed between the two lappets of the head, and opens into the cavity of the buccal mass. It is dark in colour, somewhat ovoid in shape, the posterior portion being swollen. From the ventral surface of this posterior portion the radula-sac extends backwards for a short distance below the oesophagus. There is no jaw, nor did I find any trace
of a rudiment of one. Two similar and symmetrical muscular masses project into the cavity of the buccal mass in front of the radula, one on each side of the middle line. They are dark in colour, and each is simply a muscular thickening of the wall of its respective side.
The radula is spatulate in shape, the pointed end being anterior. There are forty-four rows of teeth, the rows being set obliquely to the median
line. If the radula is mounted whole, only two kinds of teeth are visible, as it is very difficult to spread it out flat, but if separated with needles three distinct kinds can be distinguished (fig. 4)—(1) central, (2) lateral, (3) marginal.
The central tooth (tc) has a broad base, bearing a median cusp which is almost square in outline. On either side of this are a number of smaller cusps, six or seven, but the number differs with each central tooth and often on the two sides. The cusps next to the median on either side are smaller than those more remote; but all taper to a sharp point. On either side of the central tooth, and placed slightly above its upper margin, is a small elongated lateral tooth (il) which is somewhat blunt at the tip. Next to this is another lateral tooth, of larger size, which bears two cusps. The division into cusps is not the same in every tooth. Some have a large outer cusp and a very small, narrow inner one; in others the cusps are of equal length and breadth. But this difference is due probably to some being more worn away than others. The remaining teeth on each side of the laterals are the marginals (tm). They are all curved, simple, conical teeth, the tips of which are somewhat rounded.
Hutton (1879) describes only two kinds of teeth in the radula of Amphibola—median and lateral. He also says the spcies of the teeth point forward. He gives a very rough sketch of the radula, but the shape is not correct. In his second paper (1882) he redescribes the teeth. He notes that the median tooth has five or six cusps on either side, not two or three as he formerly thought; that there is a single lateral tooth, which is often divided into two and varies in shape; and that the rest of the teeth are aculeate, and increase in length towards the margin.
The form and great size of the median tooth in Amphibola seems unusual among pulmonates, for, judging from figures of radulas of other pulmonates (Bronn's Thierreich, pl. xcv)—e.g., Siphonaria, Limnaea, Planorbis, Auricula—the median tooth is much smaller and simpler than those on either side.
Perrier (1897) says that the form of the lingual teeth is related to diet: that they are obtuse and generally numerous in herbivorous molluscs, but have the form of a hook and are less numerous in carnivorous genera. The teeth in Amphibola, therefore, agree with those of other herbivorous molluscs.
A pair of salivary glands open into the buccal cavity (fig. 3, sg) near the commencement of the oesophagus. Each gland is a long, linear, yellow, sacculated structure, which passes through the nerve-collar and runs for a short distance backward beside the oesophagus. Posteriorly they taper, and are attached together and to the wall of the oesophagus.
The oesophagus extends backwards for about two-thirds the length of the body. The posterior portion lies beneath the intestinal coil, and is visible by transparency on the ventral surface of the uninjured animal. As far as the intestinal coil the oesophagus is a narrow tube, but it then dilates a little, the dilatation being marked off from the portion in front and behind it by constrictions. This specialized portion of the oesophagus is the crop (c). Behind the crop the oesophagus becomes broader, and on a level with the posterior end of the intestinal coil it bears a finger-shaped diverticulum on the right side (div). Behind this diverticulum the oesophagus becomes broader, and opens into the stomach (st), which is U-shaped, the right limb being much smaller and narrower than the left, which extends forwards towards the heart. An outgrowth of the left limb of the
stomach forms the gizzard (gz), which consists of two globular and symmetrical muscular projections separated by a muscular girdle. If the stomach be opened and its wall examined, two folds of the epithelium in the form of a pad will be seen on its floor, one behind the entrance to the gizzard and the other just in front of it. From each of these two pads a white wavy fold runs along the floor of the stomach towards the intestine. Another wavy fold is present to the right of these two.
The stomach passes into the intestine (int), which, after running underneath the aorta on the left side of the body, crosses the median line and then forms the intestinal coil. The intestine is very long, measuring in some specimens 8 ½ in. when uncoiled. It is coiled round and round the albumengland (ag) in a double spiral, the total number of complete coils being eight, only five of which are visible on the surface. It coils four times from right to left, the fourth coil crossing the middle of the albumen-gland transversely. After coiling four times in the opposite direction it runs along the right side of the first coil and passes into the rectum. The coils from left to right alternate with those from right to left. The rectum (r) runs along the right side of the body, and opens by the anus into the triangular groove already mentioned.
The extremely long coiled intestine is characteristic of herbivorous gasteropods. Amphibola has to pass through its alimentary canal enormous quantities of mud in order to obtain the vegetable matter it requires. Examination of the contents of the stomach and the mud itself shows that the food consists principally of diatoms. Several different kinds were found, the most frequent being Navicula. The faeces are deposited in long circular strings.
Hutton's drawing (1879) of the gizzard and stomach is not quite correct; and he says there are only five coils in the intestine, all reversed. He draws the triangular groove into which the anus opens as if it formed part of the wall of the rectum itself.
The digestive gland (fig. 3, dgl) is very large, occupying together with the gonad the hinder end of the body, and extending from the region of the stomach up to the apex of the visceral spire. It occupies the median portion of the spire, and lobes extend to the edge alternating with those of the gonad. It is a much-lobed gland, dark brown in colour, and when examined fresh it is seen to be dotted with numerous brown specks, the so-called entochlorophyll granules.
The duct of the liver, which appears to be single, opens into the right limb of the stomach, near its anterior end (ld).
The cells lining the lumen of the liver are long columnar cells, but they are of varying lengths, some extending a considerable distance into the cavity, others being very short. Two kinds of cells are distinguishable. (a.) Liver cells: The large cells mentioned above, as well as smaller liver cells, contain small granules, which give the yellowish-green colour to the fresh liver. They stain pink in eosin. (b.) Ferment cells: These occur in amongst the liver cells, and each has a large cavity containing a yellowish-brown granule. These entochlorophyll granules can be seen at various stages of formation, some cells containing minute granules, others granules a little larger, others again very large granules. I tried several tests for these granules, with the following results: They turned red when treated with gentian violet, turned pale green when treated with methyl green, remained brown when treated with osmic acid, and turned dark green when treated with eosin. Acetic acid had no effect; but they dissolved in caustic potash.
These entochlorophyll granules are just as numerous in a fasting animal as in one that has been feeding. The only difference I found was that the granules from a fasting animal dissolved in caustic potash at once; those in the other animals took a long time, some of them not dissolving at all.
Schneider (1902) distinguishes three kinds of cells in the liver: (a) liver cells, (b) excretory cells, (c) lime cells.
According to him, two sorts of granules occur in the “liver cells”—small liver-granules, which stain red in eosin, and large excretion granules (entochlorophyll). The “liver cells,” he says, perform a nutritive and secretory function. The “excretory cells,” he says, stain a deep black in osmic acid. The “lime cells” contain phosphate of lime.
The liver cells, as I have described, are present in the liver of Amphibola. I tested for “excretory cells” with osmic acid, but obtained no result; and of “lime cells” I could find no trace.
MacMunn (1900) regards the cells containing entochlorophyll in molluscs as “ferment cells.” He also describes “lime cells,” but finds no trace of the so-called “excretory cells.” He tested for glycogen in the liver, but obtained no results. Nor have I found any trace of glycogen in these cells in Amphibola.