198 



GASTROPODA. 



method of formation of the eyes recalls the condition in those Gastropods in 

 which they lie upon the brain (many Opisthobraochs). The eyes of Vermetus, 

 however, so far as we know, lie externally at the bases of the tentacles. 



m> 



"---'' •• <\ .«,».V»v 



"'*' ---I i ' ■ *"" 



21 



Fk;, 90.— Eyes. — A, Patella rota; />'. Trochus magus; <', Turin, creniferus ; 1>. 

 Murex brandaris (after Hilger). bg, connective tissue; ep, ectoderm; gl, vitreous 

 body ; I, lens ; ,i , optic nerve ; /<, pigmenl ; r, retina ; st, rods. 



The ontogeny of the Gastropod eye is of great interest in so far as 

 it may, at some of its stages, be compared with the adult condition 

 of the eye in various forms. Patella, for instance, has eyes which are 

 placed in the usual position, but which arc mere pit-like depressions 

 of the surface (Fig. 90 A). In llalioti.% Trochus, etc., the pit is deeper 



