Circulatory System

All Mollusca, without exception, possess a circulatory system of more or less complexity. The centre of the system is the heart, which receives the aerated blood from the breathing organs, and propels it to every part of the body. In the Scaphopoda alone there appears to be no distinct heart.

The heart may consist simply of a single auricle and ventricle, and an aorta opening out of the ventricle. From the aorta the blood is conveyed to the various parts of the body by arteries. Veins convey the blood back to the breathing organs, after passing over which it returns by the branchial or pulmonary vein to the heart, thus completing the circuit.

As regards position, the heart is situated within the pericardium, a separate chamber which in the Pelecypoda, Cephalopoda, and the bilaterally symmetrical Gasteropoda lies on the median line, while in the asymmetrical Gasteropoda it is on one or other of the sides of the body, usually the right. The veins connected with the branchiae, and consequently the auricle into which they open, are situated behind the ventricle in the Opisthobranchiata (whence their name), while in the Prosobranchiata they are situated in front of the ventricle.

The number of auricles corresponds to the number of branchiae. Thus there is only one auricle in the great majority of Prosobranchiata (which are accordingly classified as Monotocardia), and also in the Opisthobranchiata, while the Pulmonata have a single auricle corresponding to the pulmonary chamber. There are two auricles in the Amphineura, in a small group of Gasteropoda, hence known as Diotocardia, in all Pelecypoda, and in the Dibranchiate Cephalopoda. In the Tetrabranchiate Cephalopoda alone there are four auricles corresponding to the four branchiae.

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.

This progressive fusion of the mantle edges may be taken as indicating definite stages in the development of the Pelecypoda. A perfectly free mantle edge, joined at no point with the edge of the adjacent mantle, occurs in Nucula, Arca, Anomia, and Trigonia (see Fig. 80, A, B). Here there is nothing in the nature of a siphon, either anal or branchial; in other words, no contrivance exists to prevent the spent water which has passed over the branchiae from becoming mixed with the fresh water which is to reach them. When the mantle edges are fused at one point only, this is invariably on the middle part of the posterior side, thus separating off an anal opening which may become prolonged into a tube-like form. At the same time the adjacent underlying portions of the mantle edges draw together, without actually coalescing, to form an opening for the incurrent stream of water, the rudiments of the ‘branchial siphon’ (Fig. 80, C). This is the case with most Mytilidae (see Fig. 75) with Cardita, Astarte, and Pisidium. In the next stage the branchial opening is separated off by the concrescence of the mantle edges beneath it, and we have the mantle united in two places, thus forming three openings, the ventral of which is the opening for the protrusion of the foot (Fig. 80, D). This is the case in Yoldia, Leda, the majority of the Eulamellibranchiata (e.g. Lucina, Cyrena, Donax, Psammobia, Tellina, Venus, Cardium, Mactra), and all Septibranchiata. In Chama and Tridacna the fused portions of the mantle become more extended, and in Pholas, Xylophaga, Teredo, Pandora, and Lyonsia this concrescence takes place over the greater length of the whole mantle edge, so that the mantle may be regarded as closed, with the exception of the three apertures for the foot and the two siphons (Fig. 80, E).

In certain genera there occurs, besides these three apertures, a fourth, in the line of junction between the pedal and branchial orifices. It appears probable that this fourth orifice (which has been regarded by some as an inlet for water when the siphons are retracted), stands in relation to the byssal apparatus (Fig. 80, F). In Lyonsia, for instance, a thick byssus protrudes through the orifice, which is large and open. In Solen, Lutraria, Glycimeris, Cochlodesma, Thracia, Aspergillum, and a few more genera, which have no byssus, the orifice is very small and narrow. It is possible that in these latter cases, the byssal apparatus having become atrophied, the orifice has been correspondingly reduced in size.[281]

Mantle Reflected over the Shell.—It is sometimes the case that the mantle edges tend to double back over the external surface of the shell, and to enclose it to a greater or less extent. When this process is carried to an extreme, the edges of the reflected mantle unite, and the shell becomes completely internal. We see an incipient stage of this process in Cypraea and Marginella, where the bright polish on the surface of the shell is due to the protection afforded by the lobes of the mantle. A considerable portion of the shell of Scutus is concealed in a similar way, while in Cryptochiton, Lamellaria, and Aplysia the shell is more or less completely enclosed. Among Pulmonata, it is possible that in forms like Vitrina, Parmacella, Limax, and Arion, we have successive stages in a process which starts with a shell completely external, as in Helix, and ends, not merely by enveloping the shell in the mantle, but by effecting its disappearance altogether. In Vitrina and some allied genera we have a type in which the mantle lobes are partly reflected over the shell, which at the same time exhibits rather less of a spiral form than in Helix. In the stage represented by Parmacella, the mantle edges have coalesced over the whole of the shell, except for a small aperture immediately over the spire; the nucleus alone of the shell is spiral, the rest considerably flattened. In Limax the shell has become completely internal, and is simply a flat and very thin plate, the spiral form being entirely lost, and the nucleus represented by a simple thickening at one end of the plate. In Arion, the final stage, we find that the shell, being no longer needed as a protection to the vital organs, has either become resolved into a number of independent granules, or else has entirely disappeared.

Some indications of a similar series of changes occur in the Pelecypoda. The mantle edge of Lepton is prolonged beyond the area of the valves, terminating in some cases in a number of filaments. In Galeomma and Scintilla the valves are partially concealed by the reflected mantle lobes, and in a remarkable form recently discovered by Dall[282] (Chlamydoconcha) the shell is completely imbedded in the mantle, which is perforated at the anterior end by an orifice for the mouth, and at the posterior end by a similar orifice for the anus. In all these cases, except Lepton, it is interesting to notice that the hinge teeth have completely disappeared, the additional closing power gained by the external mantle rendering the work done by a hinge unnecessary. It is quite possible, on the analogy of the Gasteropoda mentioned above, and also, it may be added, of the Cephalopoda and other groups, that we have here indicated the eventual occurrence of a type of Pelecypoda altogether deprived of valves, a greatly thickened mantle performing the part of a shell.[283]

The following works will be found useful for further study of this portion of the subject:—