334 



MOLLUSCA. 



Many bivalves, especially littoral forms, are highly sensitive to light. This 

 is due to the presence of pigraented cells with a refractile cuticle on the edge of 

 the mantle ; these may be aggregated, as in Area, into groups, and so assume 

 the form of a compound eye. In Area some of the eyes have the form of pits 



FIG. 260. Axial section through the eye of Pectrn (from Lang after Patten), c cornea; 

 I lens ; ep pigmented ectoderm ; g layer of ganglion cells ; r retina ; st layer of rods of 

 retina ; d tapetum ; e pigment ; /sclerotic; n optic nerve, n 1 and H 2 its two branches. 



of the skin, while in others the sensitive surface is convex towards the light, 

 and the eye forms a slight projection on the surface : the latter are on the type 

 of compound eyes. In Pccten and Spondyhis the eyes are much more complex 

 (Fig. 260). They are placed at the end of short tentacles at the edge of the 



