Fig. 85.—Lima squamosa Lam., Naples, showing tentacular lobes of mantle (t, t); a, anus; ad.m, adductor muscle; br, br, branchiae; f, foot; sh, shell.
Taste.—The sense of taste is no doubt present, to a greater or less extent, in all the head-bearing Mollusca. In many of these a special nerve or nerves has been discovered in the pharynx, connecting with the cerebral ganglion; this no doubt indicates the seat of the faculty of taste. The Mollusca vary greatly in their likings for different kinds of food. Some seem to prefer decaying and highly odoriferous animal matter (Buccinum, Nassa), others apparently confine themselves to fresh meat (Purpura, Natica, Testacella), others again, although naturally vegetarian, will not refuse flesh on occasion (Limax, Helix).
Mr. W. A. Gain[284] has made some interesting experiments on the taste of British land Mollusca, as evidenced by the acceptance or rejection of various kinds of food. He kept twelve species of Arion and Limax, and eight species of Helix in captivity for many months, and tried them with no less than 197 different kinds of food, cannibalism included. Some curious points came out in his table of results. Amalia gagates appears to be surprisingly omnivorous, for out of 197 kinds of food it ate all but 25; Arion ater came next, eating all but 40. Limax arborum, on the other hand, was dainty to a fault, eating only seven kinds of food, and actually refusing Swedes, which every other species took with some avidity. Certain food was rejected by all alike, e.g. London Pride, Dog Rose, Beech and Chestnut leaves, Spruce Fir, Common Rush, Liverwort, and Lichens; while all, or nearly all, ate greedily of Potatoes, Turnips, Swedes, Lettuces, Leeks, Strawberries, Boletus edulis, and common grasses. Few of our common weeds or hedgerow flowers were altogether rejected. Arion and Limax were decidedly less particular in their food than Helix, nearly all of them eating earthworms and puff-balls, which no Helix would touch. Arion ater and Limax maximus ate the slime off one another, and portions of skin. Cyclostoma elegans and Hyalinia nitida preferred moist dead leaves to anything else.
II. Sight
Position of Eyes.—In the majority of the head-bearing Mollusca the eyes are two in number, and are placed on, or in the immediate neighbourhood of the head. Sometimes they are carried on projecting tentacles or ‘ommatophores,’ which are either simple (as in Prosobranchiata) or capable of retraction like the fingers of a glove (Helix, etc.). Sometimes, as in a large number of the marine Gasteropoda, the eyes are at the outer base of the cephalic tentacles, or are mounted on the tentacles themselves, but never at the tip (compare Fig. [60], p. 153 and Fig. [98], p. 199). In other cases they are placed somewhat farther back, at the sides of the neck. The Pulmonata are usually subdivided into two great groups, Stylommatophora and Basommatophora (Fig. [86]), according as the eyes are carried on the tip of the large tentacles (Helix, and all non-operculate land shells), or placed at the inner side of their base (Limnaea, Physa, etc.). In land and fresh-water operculates, the eyes are situated at the outer base of the tentacles.
Fig. 86.—A, Limnaea peregra Müll.; e, e, eyes; t, t, tentacles; B, Helix nemoralis Müll.; e, e, eyes; t, t, tentacles; p.o, pulmonary orifice.
In the Helicidae, careful observation will show that the eyes are not placed exactly in the centre of the end of the tentacle, but on its upper side, inclining slightly outwards. The eye is probably pushed on one side, as it were, by the development of the neighbouring olfactory bulb. The sense of smell being far more important to these animals than the sense of sight, the former sense develops at the expense of the latter.
Organisation of the Molluscan Eye.—The eye in Mollusca exhibits almost every imaginable form, from the extremely simple to the elaborately complex. It may be, as in certain bivalves, no more than a pigmented spot on the mantle, or it may consist, as in some of the Cephalopoda, of a cornea, a sclerotic, a choroid, an iris, a lens, an aqueous and vitreous humour, a retina, and an optic nerve, or of some of these parts only.