ORGANISATION OF THE EYE 



i8i 



p. 199). In other cases they are placed somewhat farther Ijack, at 

 the sides of the neck. The Puhnonata are usually suljdivided into 

 two great groups, Stylommato'pliora and Basommatoi^liora (Fig. 

 86), according as the eyes are carried on the tip of the large 

 tentacles {Helix, and all non-operculate land sliells), or placed at 

 the inner side of their l)ase {Lhnnaea, Plujsa, etc.). In land and 

 fresh-water operculates, the eyes are situated at the outer base of 

 the tentacles. 



In the Helicidae, careful observation will sliow 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, l)y tlie development of 

 the neighl)ouring olfactory bulb. The sense of smell lieing far 

 more important to these animals than the sense of sight, the 

 former sense develops at the expense of the latter. 



Organisation of the MoUuscan Eye.- — Tlie eye in Mollusca 

 exhibits almost ever}' imaginalile form, 

 from the extremely simple to the 

 elaborately complex. It may l)e, as i]i 

 certain l)ivalves, no more than a, pig- 

 mented 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 acpieous and vitreous 

 luunour, a retina, and an optic nerve, or 

 of some of these parts only. 



In most land and fresh-water Mol- '' 

 lusca the eye may be regarded, roughly 

 speaking, as a ball connected l)y an 

 exceedingly fine thread (the optic nerve) ^^^ ^~ 

 with a nerve centre (the cerebral gang- 

 lion). In PcducUna this ball is elliptic, 

 in PlanorMs and Neritina it is drawn 

 out at the back into a conical or pear 

 shape. In Helix (Fig. 87) there is a. structureless meml)rane, 

 .siuTounding tlie whole eye, a lens, and a retina, the latter 

 consisting of a nervous layer, a cellular layer, and a layer of 

 rods containing pigment, tliis innermost layer (that nearest the 

 lens) being of the thickness of lialf the whole retina. 



Comparing the eyes of different Gasteropoda together, we find 



—Eye of Helix pmnatia 

 L., retracted within the 

 tentacle ; c. cornea ; c}), epi- 

 thelial layer; I, lens; op.v, 

 optic nerve ; /•. retina. (After 

 Simroth.) 



