the respiratory cavity containing the gills, but it also serves to enclose the body so that the latter is surrounded by the shell, from which the head and foot can be protruded at the will of the animal.
The shell consists of an organic basis the substance of which is called conchiolin, impregnated with carbonate of lime, with a small proportion, 1–2%, of phosphate of lime. On the outside of the shell is a non-calcified layer of conchiolin called the periostracum, secreted by the thickened edge of the mantle. The zone of the external surface of the mantle within the edge secretes a layer formed of prisms of calcite; the rest of the epithelium from this zone to the apex secretes the inner layer of the shell, composed of successive laminae; this is the nacreous layer, and in certain species has a commercial value as nacre or mother-of-pearl. Thus the growth of the shell in extent is due to additions to the prismatic layer at the edge, its growth in thickness to new layers of nacre deposited on its inner surface. In many cases in various classes the mantle is reflected over the edges of the shell, so as to cover more or less completely its outer surface. When this covering is complete the shell is contained in a closed sac and is said to be “internal,” but the sac is lined by ectoderm and the shell is always morphologically external. In one or two cases the epithelium of the foot secretes a calcified shell, which is either free as in Argonauta or adherent as in Hipponyx.
The ctenidia (fig. 1) are the branchial organs of the Mollusca. In the primitive condition there is one on each side in the mantle cavity, towards the posterior end of the body. Each is an outgrowth of the body-wall at the side of the body, and consists of an axis containing two main vessels, an afferent and efferent, and bearing on either side a series of transverse plates whose blood-sinuses communicate with the vessels of the axis. The afferent vessel of the ctenidium receives blood from the vena cava or principal blood-sinus of the body, the efferent vessel opens into the auricle of its own side. Near the base of the ctenidium is a patch of sensory epithelium innervated from the branchial nerve, forming a sense-organ called the osphradium, whose function is to test the water entering the branchial cavity. The branchial current is maintained by the cilia which cover the surface of the ctenidia, except in Cephalopoda, in which cilia are absent and the current is due to muscular action. Thus in the primitive mollusc the mantle-cavity contains a symmetrical group of structures at the posterior end of the body, and this group of structures is called the pallial complex. It consists of the anus in the middle, a renal organ and renal aperture on each side of this, and a ctenidium outside or anterior to the renal organ, an osphradium being situated at the base of the ctenidium.
Internal Anatomy: Digestive Tube.—In primitive Mollusca the mouth and anus are the two extremities of the body, but the anus may be brought to an anterior position by a ventral flexure, complicated in Gastropoda by a lateral torsion. The alimentary tube consists of three regions: firstly, the anterior buccal mass with the oesophagus, of ectodermic origin, and therefore bearing cuticular structures, namely the jaws and radula; secondly, the mid-gut, of endodermic origin and including the stomach and liver; and, thirdly, the hind-gut or intestine. The radula consists of a chitinous band bearing teeth, secreted by a ventral caecum of the pharnyx and moved by an apparatus of cartilage and muscles. It was present in the ancestral mollusc, occurs in nearly all archaic types, and is only absent in the most specialized forms, in which it has evidently been lost; these forms are certain Neomeniomorpha, all the Lamellibranchia, various degenerate Gastropoda, and the Cirrhoteuthidae among Cephalopods. The teeth are secreted by a small number of cells at the closed end of the caecum, the basal membrane by a transverse row of cells in front of these. The teeth are disposed in transverse rows, and in each row they are arranged symmetrically on either side of a central tooth. In Polyplacophora there are eight on each side (8.I.8); in Scaphopoda two on each side (2.I.2); in almost all Cephalopoda three on each side (3.I.3); in Gastropoda the number varies very much in different subdivisions. Beneath the anterior parts of the radula where it emerges from the caecum are a pair of cartilages, and attached to these a number of special muscles by which the radula is moved backwards and forwards to act as a rasp. The secretion of the radula at the closed end of the caecum is continuous, so that it is constantly growing forward as fast as its exposed anterior portion is worn away by use, just as a fingernail is pushed forward by constant growth at its posterior end, and is worn away or has to be cut short from time to time at its outer end.
Circulation.—The system of blood-vessels is entirely separate from the coelomic cavities. It consists of arteries, veins and sinuses, but ramified capillaries are usually absent except in the integuments of Cephalopods. The arteries and veins have proper endothelial walls; they pass abruptly into the sinuses and in some cases communication is effected by orifices in the walls of the vessels, as for example in the vena cava of Nautilus. The heart is situated in the pericardium on the dorsal side of the intestine and at the posterior end of the animal. The pericardium never contains blood, as is well shown in those forms which have red corpuscles in their blood; these corpuscles are never found in the pericardium.
The heart receives blood from the gills and mantle, and pumps it through arteries to the body. It consists of a median ventricle with muscular walls and a cavity traversed by muscular strands. On either side of the ventricle, in the primitive condition, is a thin-walled auricle, opening into the ventricle by a valved opening. Each auricle forms the terminal enlargement of the efferent vein of the ctenidium of its own side. In Nautilus two pairs of auricles are present, corresponding with the two pairs of ctenidia. In the primitive form a single anterior aorta is given off from the ventricle, the two together representing the dorsal blood-vessel of Chaetopods. In more specialized forms a posterior aorta passes backwards from the ventricle, as in Gastropods and the majority of Lamellibranchs. The ramifications of the arteries convey the blood to all parts of the body, and it finally reaches the venous sinuses, the chief of which are the pedal, the pallial and the median-ventral. The last is between the pericardium and the foot; from it the blood passes through the renal organs to the ctenidia. Some blood, however, enters the auricles directly from the mantle, without passing through the ctenidia. In the majority of Gastropoda one gill and one auricle are lost.
The blood is usually a colourless liquid containing amoeboid cells and sometimes other corpuscles called haematids. It may be coloured blue by haemocyanin, a respiratory compound containing copper. In a few forms the blood contains haemoglobin, either in solution or in haematids (red blood-corpuscles). In the Gastropoda the muscular tissue of the buccal mass is coloured red by haemoglobin.
Nervous System.—The central nervous system may be described as consisting of a collar surrounding the oesophagus, and two pairs of cords arising from the collar and passing backwards. The two pairs of cords arise from the same point of the collar. The ventral cords are the pedal, the dorso-lateral, the pleural, the former innervating the foot, the latter the mantle. The dorsal half of the collar is the cerebral commissure, the ventral the labial commissure. The pedal cords are connected by commissures, and the pedal and pleural of each side are similarly connected. The pallial cords are united to one another posteriorly, dorsal to the rectum. This is the condition of the nervous system found in Chiton and the other Amphineura, but may not be in all respects the ancestral condition. Generally the system is differentiated into ganglia connected by nerve-cords consisting of nerve-fibres only. At the point of the collar whence the nerve-cords arise are the cerebral ganglia; from these one pair of connectives passes to a pair of pedal ganglia, and another pair of connectives to a pair of pleural ganglia. Pedal and pleural on each side are connected by a pleuro-pedal connective. Each pleural ganglion gives off a long nerve which supplies the viscera, and the two unite posteriorly below the intestine. There are usually three small ganglia on the course of this visceral commissure, namely, the right and left visceral ganglia and the abdominal. The perioesophageal nerve-ring of Chaetopoda and Arthropoda is represented, not by the collar first mentioned in the above description, but by the commissures connecting the cerebral and pedal ganglia. The labial commissure supplies only the buccal mass and the oesophagus and stomach.
The special sense-organs are a pair of eyes on the head, a pair of otocysts or statocysts, and a pair of osphradia which have already been mentioned. In certain cases accessory eyes are also present, e.g. the pallial eyes of Pecten and other Lamellibranchs, and of Chitons. The otocysts are invaginations of the epithelium of the foot, but are innervated from the cerebral ganglia, and the same innervation has been proved in some cases for the osphradia.
Reproduction and Development.—Molluscs are usually of separate sexes, but sexual dimorphism is seldom highly developed. Hermaphroditism is secondary, and occurs in one sub-class of Gastropoda, in some Lamellibranchs, and in one sub-order of Amphineura. In Cephalopods and the majority of Gastropods copulation occurs. As a rule no parental care is exhibited, but incubation of the developing ova within some part of the parental body, or receptacles attached to the parent, occurs in some Lamellibranchs, some Gastropods, and in Argonauta among the Cephalopods. True viviparity, that is the development of the ova within the oviduct, is very rare, occurring only in one case among the Amphineura and in some aquatic and pulmonate Gastropoda.
The egg-cell of Mollusca is either free from food-material—a simple protoplasmic corpuscle—or charged with food-material to a greater or less extent. Those cases which appear to be most typical—i.e. which adhere to a procedure which was probably common at one time to all then existing Mollusca and has been departed from only
