Two kidneys open on either side of the anus. The genital gland is large, occupying nearly all the posterior part of the body, the sexual products being emitted through the right kidney. The veliger has already been figured (p. 131, Fig. [44]). The embryonic shell is formed of two calcareous laminae, which subsequently unite to form the tube.
With regard to their general relationships, the Scaphopoda resemble the Gasteropoda in their univalve shell, and in the possession of a radula; while the pointed foot, the non-lobed velum in the veliger, the generative system, the bilateral symmetry of the organs generally, and the absence of any definite head, eyes, or tentacles, are points which approximate them to the Pelecypoda.
The Scaphopoda are known from Devonian strata to the present time. They are found at a depth of a few fathoms to very deep water. The only three genera are Dentalium, Siphonodentalium (subg. Cadulus), and Pulsellum, which differ in the structure of the foot, as described above.
CLASS PELECYPODA
Cephalic region rudimentary, mantle consisting of two symmetrical right and left lobes, covering the body and secreting a bivalve shell hinged at the dorsal margin; no radula, sexes usually separate. Reference has already been made to the reproductive system (p. [145]), breathing organs (p. [164] f.), mantle (p. [172]), nervous system (p. [205]), digestive system (p. [237] f.), and nomenclature of the various parts of the shell (p. [269] f.).
The shape of the shell, in many Pelecypoda, involving as it does the position, size, and number of the adductor muscles, is probably due to mechanical causes, depending on the habits and manner of life of the individual genus. Thus in a typical dimyarian or two-muscled bivalve, e.g. Mya (Fig. [300], A), the adductor muscles lie well towards each end of the long axis of the shell, with the hinge about midway between them. In this position they are best placed for effectually closing the valves, and since they are nearly equidistant from the axis of motion, i.e. from the hinge, they do an equal amount of work, and are about equal in size. But in a form like Modiola, where the growth of the shell is irregular in relation to the hinge-line, the anterior muscle is brought nearer and nearer to the umbones, where its power to do work, and therefore its size, becomes less and less. But the work to be done remains the same, and the posterior muscle has to do it nearly all; hence it moves farther and farther away from the hinge-line, and at the same time gains in size. In shells like Ostrea, Pecten, and Vulsella, the anterior muscle, having drawn into line with the hinge and the posterior muscle, becomes atrophied, while the posterior muscle, having double work to do, has doubled its size.[409]
Fig. 300.—Illustrating changes in the position and size of the adductor muscles according to the shape of the shell: A, Mya; B, Modiola; C, Vulsella. The upper dotted line shows the hinge-line, the lower connects the two muscles.
The development of the foot, again, largely depends upon habits of life. It is well developed in burrowing forms, while in sessile genera (Ostrea, Chama, Spondylus) it becomes unnecessary and aborts. Even in Pecten, which does not become sessile, but has ceased to use the foot as an organ of progression, a similar result follows. Forms which burrow deeply often “gape” widely, sometimes at one end only, sometimes at both. Venus, Donax, Tellina, Mactra, which are shallow burrowers, do not gape; Solen, Lutraria, and to a less degree Mya, burrow deeply and gape widely. In order to burrow deeply the foot must be highly developed, and the larger it becomes, the more will it tend to keep the valves apart at the place where it is habitually protruded. Burrowing species always remain in communication with the surface by means of their siphons, the constant extension of which tends to keep the valves apart at the end opposite to the foot. Burrowing species, again, tend to burrow in such a way as to descend most easily, and not be impeded by their own shells; in other words, they act as a wedge, and descend with their narrowest part foremost. But the burrowing organ, the foot, has to follow suit, and gradually draws round to the narrowest part of the shell, so that the habitual deep burrower, such as Lutraria, lies with its long axis exactly at right angles to the surface, its siphons protruding from, and keeping open, the uppermost or posterior margin of the shell, and the foot producing the same effect upon the lower or anterior margin. The deeper the burrower, the more elongated does the shell become, until, through forms like Pholas and Saxicava, we arrive at Solen, the most highly specialised burrower of all, in which the breadth of the shell is equal throughout, and no obstructive curve exists to impede its rapid ascent or descent.
The Pelecypoda have been classified in various ways; by the completeness or sinuation of the pallial line, depending on the absence or presence of siphons, by the number of adductor muscles, by the character of the hinge-teeth, and by the number of the branchiae. For various reasons, none of these methods have proved entirely satisfactory. That adopted here was suggested by Pelseneer, and depends upon the character of the branchiae themselves, as suggesting successive stages of development (p. [166] f.).