Fam. 14. Vaginulidae.—Radula, p. [234]; animal slug-like, covered with a coriaceous mantle, lower tentacles bifid, genital orifices widely separated, male behind the lower right tentacle, female on inferior median part of right side, anus and pulmonary orifice nearly terminal; shell absent. Single genus, Vaginula (= Veronicella).
Fig. 298.—Achatina zebra Lam., S. Africa. × ½.
Fam. 15. Onchidiidae.—Body oval, mantle thick, often warty, sometimes set with “eyes” (p. [187]), two tentacles, genital orifices widely separate, anus and pulmonary orifice as in Vaginula; no shell. Genera: Peronia, Onchidium, Onchidiella. The family appears to be an instance of Pulmonata reverting to marine habits of life.
CHAPTER XVI
CLASSES SCAPHOPODA AND PELECYPODA
CLASS SCAPHOPODA
Head rudimentary, mantle edges ventrally concrescent, forming a tube opening before and behind, and covered with a shell of the same shape; sexes separate.
Fig. 299.—Anatomy of Dentalium: a, anterior aperture of mantle; f, foot; g, genital gland; k, kidney; l, liver. (After Lacaze-Duthiers.)
The Scaphopoda form a small but very distinct class, whose organisation is decidedly of a low type. The body is usually slightly curved, the concave side being the dorsal; muscles near the posterior end attach the body to the shell. The foot, which can be protruded from the anterior or wider aperture, is rather long, pointed, and has sometimes two lateral lobes (Dentalium), sometimes a terminal retractile disc (Siphonodentalium), sometimes a retractile disc with a central tentacle (Pulsellum). The cephalic region, as in Pelecypoda, is covered by the mantle. The mouth is situated on a kind of projection of the pharynx; the buccal mass, containing the radula (p. [236]), is at the base of the foot, and the intestine branches forward from the front part of the stomach. The liver (Fig. [299]) is paired, and consists of a number of symmetrical, radiating coeca. There are no eyes, but on each side of the mouth are small bunches of exsertile filaments (captacula), which appear to act as tactile organs for the seizing of food. There is no special respiring apparatus, heart or arterial system, breathing being conducted by the walls of the mantle. The nervous system has already been described (p. 205).