VII 



ORGANISATION OF THE EYE 



l8l 



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 (^Li7nnaea^ 

 Physa, etc.). In land and fresh-water operculates, the eyes are 

 situated at the outer base of the tentacles. 



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 elab- 

 orately complex. It may be, as in cer- 

 tain bivalves, no more than a pigmented 

 spot on the mantle, or it ma}' 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 tliese parts only. 



In most land and fresh-water Mol- 

 lusca 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 gang- 

 lion). In Paliidina this ball is elliptic, 

 in Planorhis 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 con- 

 sisting 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. 



Comparing the eyes of different Gasteropoda together, we 



^opjl 



Fig. 87. — Ej^e of Helix poma- 

 tia L., retracted within the 

 tentacle ; c, cornea ; ep, 

 epithelial layer; I, lens; 

 op.n, optic nerve ; r, ret- 

 ina. (After Simroth.) 



