A single aorta occurs only in the Amphineura and in the Tetrabranchiate Cephalopoda. In all the other groups there are two aortae, leading out of the anterior and posterior ends of the ventricle in Pelecypoda and Dibranchiate Cephalopoda, while a single aorta leads out of the posterior end alone, and subsequently bifurcates, in most of the Gasteropoda. One aorta, the cephalic, supplies the front part of the body, the oesophagus, stomach, mantle, etc.; the other, the visceral aorta, supplies the posterior part, the liver and sexual organs.
The general circulatory system in the Mollusca has not yet been thoroughly investigated. As a general rule, the blood driven from the ventricle through the aorta into the arteries, passes, on reaching the alimentary canal and other adjacent organs, into a number of irregular spaces called lacunae. These in their turn branch into sinuses, or narrow tubes covered with muscular tissue, which penetrate the body in every direction. In the Dibranchiate Cephalopoda true capillaries are said to occur, which in some cases form a direct communication between the arteries and veins. According to some authorities[277] capillaries and veins exist in certain Pelecypoda in connexion with the intestinal lacunae, but this again is regarded by others as not established. A similar difference of opinion occurs with regard to the precise function of the foot-pore which occurs in many Mollusca, some holding that it serves as a means for the introduction of water into the blood-vascular system, while others regard it as a form of secretion gland, the original purpose of which has perhaps become lost.
Blood.—As a rule, the blood of the Mollusca—i.e. not the corpuscles but the liquor sanguinis—is colourless, or slightly tinged with blue on exposure to the air. This is due to the presence of a pigment termed haemocyanin, in which are found traces of copper and iron, the former predominating. Haemoglobin, the colouring matter of the blood in Vertebrates, is, according to Lankester,[278] of very restricted occurrence. It is found—(1) in special corpuscles in the blood of Solen legumen (and Arca Noae); (2) in the general blood system of Planorbis; (3) in the muscles of the pharynx and jaws of certain Gasteropoda, e.g. Limnaea, Paludina, Littorina, Chiton, Aplysia. This distribution of haemoglobin is explained by Lankester in reference to its chemical activity; whenever increased facilities for oxidisation are required, then it may be present to do the work. The Mollusca, being as a rule otiose, do not possess it generally diffused in the blood, as do the Vertebrata. The actively burrowing Solen possesses it, and perhaps its presence in Planorbis is to be explained from its respiring the air of stagnant marshes. Its occurrence in the pharyngeal muscles and jaws of other genera may be due to the constant state of activity in which these organs are kept.[279]
According to Tenison-Woods[280] a species of Arca (trapezia Desh.) and two species of Solen, all Australian, have red blood. It is suggested that in these cases the habits of the animal (the Solen burrowing deeply in sand, the Arca in mud) require some highly oxidising element, surrounded as the creature is by ooze. In Arca pexata (N. America) the blood is red, the animal being familiarly known as the ‘bloody clam.’ Burrowing species, however, are not all distinguished by this peculiarity. Tenison-Woods finds red fluids in the buccal mass of many Gasteropoda, e.g. in species of Patella, Acmaea, Littorina, Trochus, Turbo, giving the parts the appearance of raw meat.
The Mantle
On the dorsal side of the typical molluscan body, between the visceral sac and the shell, lies a duplicature of the integument, generally known as the mantle. The depending sides of the mantle, which are usually somewhat thickened, enclose between themselves and the body mass a chamber of varying size and shape, called the mantle cavity, which communicates freely with the external air or water, and encloses and furnishes a protection for the organ or organs of respiration. On its upper or dorsal surface the mantle is closely applied to the shell throughout its whole extent, the cells with which it is furnished secreting the materials from which the shell is formed (see p. [255]). The whole mantle is capable, to some degree, of secreting shelly matter, but the most active agent in its production is the mantle edge or margin.
In the Prosobranchiata the mantle cavity, for reasons which have already been explained, is found on the left side of the animal, its front portion being in many cases produced into a tubular siphon. Within the mantle cavity are found, besides the branchia, the anus, the apertures of the kidneys, and the osphradium. In the pulmonata the mantle fold encloses a so-called lung-cavity. The front edge of the mantle coalesces with the integument of the neck in such a way as to enclose the cavity very completely, the only communication with the outer air being by means of the contractile breathing or pulmonary aperture on the right side. In the Tectibranchiate Opisthobranchs the mantle fold is inconsiderable, and is usually not of sufficient extent to cover the branchia, while in the Nudibranchs, which have no true branchiae, it disappears altogether.
In the Pelecypoda the mantle cavity is equally developed on each side, enclosing the two sets of branchiae. The mantle may thus be regarded as consisting of two equal portions, which form a sort of lining to the two valves. The lower or ventral portion of the mantle edges may be simple, or provided with ocelli (Pecten, Arca), tentacles, cilia (Lima, Lepton), or doubled folds. The two portions of the mantle touch one another along the whole line of the edge of the two valves, and, although thus in contact, may remain completely separate from one another, or else become permanently united at one or more points. This fusion of the mantle edges corresponds to important changes in the organisation of the animal as a whole. The anal and branchial siphons are no more than prolongations of the mantle edges on the posterior side into a tubular form. These ‘siphons’ exhibit the siphonal form more distinctly according as the adjacent portions of the mantle become more definitely fused together.
Fig. 80.—Diagram illustrating the various stages in the closing of the mantle in Pelecypoda: A, mantle completely open; B, rudiments of siphons, mantle still completely open; C, mantle closed at one point; D, mantle closed at two points, with complete formation of siphonal apertures; E, development of siphons, ventral closure more extended; F, mantle closed at three points, with fourth orifice: f, foot; s.a, s.b, anal and branchial siphons; 1, 2, 3, first, second, and third points of closure of mantle. (After A. Lang.)