THE EYES OF GASTEROPODS 



159 



differ, however, in that the mouth of the pit has become constricted by 

 the ingrowth of a circular fold of the cuticular epithelium, which reduces 

 the size of the opening to a small pupillary aperture (Fig. 34, Chap. 3, 

 p. 48). Moreover, the interior of the vesicle is completely filled with 

 a clear, gelatinous material which is apparently secreted by the cells 

 which line the vesicle. This is described as the vitreous humour. 



3. Murex. — The ornamental spiral shells of this genus have long 

 tapering spines and are commonly known as " venus's combs." In this 

 type the eyes are more highly organized than in the former examples, the 

 mouth of the pit has become closed and the pit is constricted off 

 from the cuticle so as to form a vesicle (Fig. 35, Chap. 3, p. 49). The 



.'tent 



g'nM 



Fig 



B 



117. — A: Veliger Larva of Patella ccerulea viewed from the Left 

 Side showing Ocelli on Apical Plate. B : Dorsal View of Acmaa 

 virginea, just after metamorphosis, in which the ocelli are seen to 

 be covered by the margin of the shell. the larval shell is retained 

 as an Apical Knob, which will be cast off later. (Redrawn from 

 MacBride : A, after Patten ; B, after Boutan.) 

 ft. : foot. oc. : ocellus. 



gl. : glands in roof of mantle. p.g. : rudiment of pedal ganglion. 



/. sh. : larval shell. tent. : tentacle. 



me. : mantle cavity. vel. 



mf.: 



muse. 



mantle fold. 

 : muscle. 



vh. 



velum, 

 visceral hump. 



outer lamina of the circular cuticular fold joins over the superficial aspect 

 of the vesicle and forms with a layer of ingrowing mesenchyme what is 

 termed the " secondary cornea," whereas the outer part of the epithelial 

 wall of the optic vesicle, which remains thin and transparent, is called the 

 " primary cornea." The remaining inner and larger part of the epithelial 



