156 THE PINEAL ORGAN 



are separated by a cleft from a tapetum and a pigment layer. The whole 

 is enclosed by a fibrous sheath surrounded by a loose mesenchyme 

 containing blood-vessels. The cellular lens is highly convex on its deep 

 surface, slightly convex superficially, where it is covered by a transparent 



Fig. 114. — Trochophore Larva of Patella coerulea. 



Ventral side showing : ap., apical plate ; blp., blastopore ; /., rudiment of foot ; 

 set., setse. Compare with Figs. 58, 108. 

 (After Patten, redrawn from MacBride.) 



cornea. This consists of a thin cuticle covering a layer of hypoderm cells 

 beneath which is a fibrous stratum continuous with and forming part of the 

 fibrous sheath enclosing the eyeball. The clear hypoderm cells of the 

 cornea are continuous circumferentially with the surrounding hypoderm 

 cells which are deeply pigmented and thus function as a fixed iris. 



The only cephalic eyes which occur in the class Lamellibranchiata are 

 a pair of ocelli which are found in the bases of the most anterior filament 

 of the inner gill lamina in Myttlus, the common sea- water mussel, and some 

 allied genera. 



The Eyes of Amphineura 



With the exception of the Chitons, or " coat-of-mail shells " (Fig. 115), 

 the Amphineura are a lowly organized class of mollusc in which in the 

 adult animal the region of the head is hardly distinguishable from the body, 

 and it is neither provided with eyes nor tentacles. 



The Chitons are characterized by a series of eight overlapping valves, 



