=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. [Illustration: 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. In most land and fresh-water Mollusca the eye may be regarded, roughly speaking, as a ball connected by an exceedingly fine thread (the optic nerve) with a nerve centre (the cerebral ganglion). In _Paludina_ this ball is elliptic, in _Planorbis_ and _Neritina_ it is drawn out at the back into a conical or pear shape. In _Helix_ (Fig. 87) there is a structureless membrane, surrounding the whole eye, a lens, and a retina, the latter consisting of a nervous layer, a cellular layer, and a layer of rods containing pigment, this innermost layer (that nearest the lens) being of the thickness of half the whole retina. [Illustration: FIG. 87.--Eye of _Helix pomatia_ L., retracted within the tentacle; _c_, cornea; _ep_, epithelial layer; _l_, lens; _op.n_, optic nerve; _r_, retina. (After Simroth.)]