I82 



STAGES OF DEVELOPMENT IX THE EYE 



CHAP. 



that they represent stages in a general course of development. 

 Thus in Patella the eye is scarcely more than an invagination 

 or depression in the integument, which is lined with pig- 

 mented and retinal cells. The next upward stage occurs in 

 TrocJi'US, where the depression becomes deeper and hladder- 

 shaped, and is filled with a gelatinous or crystalline mass, l:)ut 

 still is open at the top, and therefore permits tlie eye to he 



Fig. 88. — Eyes of Gasteroi:)oda, showing arrest of development at successive stages : A, 

 Patella; B, Trochus ; C, Turbo; D, JIurex ; ep, epidermis; I, lens; op.n. optic 

 nerve ; r, retina ; v.h, vitreous liumour. (After Hilger. ) 



Lathed in water. Then, as in Turho, the hladder Ijecomes 

 closed l)y a thin epithelial layer, which finally, as in some Murex, 

 becomes much thicker, while the ' eyeball ' encloses a lens (Fig. 

 88), which probably corresponds with the 'vitreous humour' of 

 other types. 



In Nautilus the eye is of a very simple type. It consists of 

 a cup-shaped depression, with a small opening which is not 

 quite closed by the integument. Tlie retina consists of cells 



