Alimentary System, (pl. vii.)
Mandibles (fig. 2).—Within the annular inner lip already noticed, are two vertical conchiolin jaws forming the beak, the posterior jaw overlapping the anterior one. The exposed parts are reddish-brown in colour, while the covered parts are transparent and colourless. Each jaw consists of an uncus, alæ and apophysis, the alæ being backward and lateral expansions of the uncus.
In the anterior or upper-jaw the uncus runs forward in a decurved line, terminating in a sharp point. The outer border of the alæ forms a continuous curve with this line, and runs backward to about half the length of the apophysis, from which it stands out prominently. The lower border of the alæ is a concave curve; and the front border, from the margin of the uncus, presents a waved outline. The apophysis extends downwards, its greatest length being at its outer margin. It is fully twice the size of uncus and alæ. Its inner border forms a convex curve, which extends round to the under-border, where it becomes concave.
In the posterior or under-jaw the uncus is not so large as that of the upper-jaw, is more obtuse, and is proportionately stouter. The alæ extend
but a short distance backwards on the outer aspect, being here shortened by a concave border. At the sides they expand and become irregularly oblong in shape. They have a somewhat irregularly waved outline and extend forward beyond the uncus, so as to cover the inner portion of the alæ of the upper-jaw. The apophysis extends outwards and backwards with a slight curve, its shortest length being at the median line, where the margin is concave. Its greatest length is within, where the margin is convex. The apophysis slopes rapidly from the median line, leaving a well marked keel.
(Note.—In describing the jaws I have not adhered to the rule of speaking of the animal as though it were in a walking position, but have noticed them as they are detached and shown in the drawing.)
Taste-organ (fig. 3 a).—Within the jaws and on the posterior side of the mouth is the gustatory organ, showing a fairly well-marked division into two lobes. It is soft and uncurved, and under the microscope shows interlacing fibres.
Odontophore (fig. 3 b).—Next follows the broad radular band which works in and out of a socket in the centre of a raised papilla. It is armed with seven rows of silicious teeth. Its upper part is expanded, and has the margins recurved. The lower part has the margins incurved so as to form a cylinder, and it is probable that this part is but little used in mastication. The teeth on the upper part are stronger and stouter.
The central row of teeth consists of slightly curved, rather stout and obtuse spines, whose bases develope short obtuse prominences, one on each side, giving the denticles of this row a 3-fid appearance. The first lateral row consists of slightly curved spines, somewhat more acute than those of the median row. The spines have the base also developed as in the median row, but the inner basal spine is somewhat shorter than the outer. The next two rows consist of longer curved spines without basal spinules.
Faucial follicles (fig. 3 d). Next come the two “faucial follicles.” They are attached along one side, starting at the tongue and continuing to the commencement of the œsophagus. They are very well developed. The free margin is straight, and when folded over they form a covered channel over the radular socket.
Salivary glands.—The mandibles, tongue, odontophore, and faucial follicles are contained in the buccal mass, which narrows into the œsophagus. Outside the buccal mass, at the commencement of the œsophagus, are two slightly-raised papillæ, the lingual salivary glands. Further along the œsophagus, and imbedded in the liver, are two salivary glands with well-marked ducts.
OEsophagus (fig. 1 c).—The buccal mass narrows into the œsophagus, which passes through the nerve-collar and peri-œsophageal cartilaginous ring, and is continued to the stomach without ingluvial dilatation. It dilates slightly as it enters the stomach.
Stomach (fig. 1 m).—The stomach is saccular and thick-walled. It shows two slight constrictions. The walls present on their inner surface strong longitudinal plicæ, which are prolonged into the intestine, and continue throughout its length.
Lying within the stomach is a loose tunic or sac with thick, though almost transparent, walls, showing longitudinal plicæ or corrugations. It has a wide orifice at one end, and at the other is thin or open. I have not been able to discover that it has any organic connection with the walls of the stomach, but as I have found it in all the specimens I have examined, I do not think its occurrence can be accidental.
The pyloric opening is at the lower end of the stomach and close to the cardiac opening. The commencement of the intestine is shown by a well-marked constriction. Neither cardiac nor pyloric opening is protected by a valve.
Pyloric cœcum (fig. 1 n).—Immediately following the constriction there is given off a long cœcal dilatation with thin but tough transparent walls. It expands at its attached end and tapers gradually to its opposite rounded extremity. When in sitû it forms a half curve round the stomach. As will be presently noticed, the hepatic ducts open into this cœcum. On the broadest part of its wall, at the attached end is a circular coat formed by a radiating mass, whose nature I have not discovered.
Intestine (fig. 1 h).—From the pyloric cœcum the intestine narrows gradually until the anus is reached. At somewhat less than half the distance between the cœcum and the anus it is folded over so as to form a distinct flexure. The anus has two stalked and leaf-like lateral valves.
Ink-sac (fig. 1 h).—The ink-sac is large and broad. Its highest and broadest part lies near the intestinal flexure, and from this part it narrows to its opening into the anus. Its coat is silvery and, in places, iridescent; and the dark sepia shows through the sac-walls. Throughout its length it is held closely to the intestine by a membrane.
Liver (fig. 1 f).—The liver is large, extending from immediately above the cephalic cartilage for about two-thirds of the length of the œsophagus. It is encased in a capsule showing under the miscroscope close fibres and yellow concretions. The liver itself is loose, and under the microscope shows clustered follicles and interlacing tubes with abundant yellow concretions. The bilobed condition is not observable.
The hepatic ducts are two, and open close together into the commencement of the pyloric cœcum. They have developed upon them the light-coloured spongy “pancreatic” glands, which show under the microscope a loose fibrous tissue interspersed with yellow concretions (fig. 4). The fibrillar tissue is more loose and the yellow concretions larger, but less numerous, than in the liver. There is everywhere a network of ramifying tubes.
The whole of the organs are enclosed in a peritoneal membrane, which sends three mesenteric reflexions to the mantle, one of the arterial branches from the posterior aorta running along the anterior border of each reflexion.