440 



MOLLUSCS AND BRACHIOPODS. 



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 re- 

 garded, roughly speaking, as a ball connected by an exceedingly 

 fine thread (the optic nerve) with a nerve-centre (the cerebral 

 ganglion) In Helix (Fig. i) there is a structureless 



op.n 



2. — Eyes of Gasteropoda, showing arrest of development of successive 

 A, Patella; B, Trochus ; C. Turbo; D, Murex ; ep., epidermis; 



Fig. 

 stages : 

 /., lens ; oj). n., optic nerve ; r., retina; v.h., vitreous humour (after Hilger). 



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. 



Comparing together the eyes of different Gasteropoda, we find 

 that they represent stages in a general course of development. 



