THE OCELLI. 



331 



eyes found in certain Molluscs (Patella). It is a simple pit-like 

 depression of a thickened part of the hypodermis. The elongate 

 cells which compose the wall of this depression are arranged in a 

 simple layer and, at their free ends, which are turned to the optic 

 pit, carry a striated cuticular margin (c), while their inner or basal 

 ends give off the nerve-fibres which unite to form the common 

 optic nerve. 



Wi 13 ' T" ; ' 



According to Patten, 

 this apparently simple 

 rudiment has arisen by 

 the fusion of at least 

 four distinct pits which 

 represent primary em- 

 bryonic organs, and in 

 structure recall the eye- 

 pits on the margin of the 

 mantle in Area. The 

 nerve correspondingly 

 shows its composition 

 out of four originally 

 separate bundles. 



In later stages, the 

 eye-pit closes towards 

 the exterior (Fig. 162 

 B), the marginal parts 

 pushing inward until 

 they meet over the 

 deeper parts. In this 

 way the pit-like rudi- 

 ment gives rise to an 

 eye-cup which has by 

 this process become 

 bilaminar. The cen- 

 tral part of the outer 

 or superficial layer (I) 

 becomes the lentigen 

 layer (vitreous body), 

 while the peripheral 

 parts become the pig- 

 mented iris. The cuticular margin of these cells gradually gives rise 

 to the cuticular chitinous lens (cl) of the ocellus. Laterally, the 

 superficial layer of the eye passes direct into the hypodermis (/*). 



The deeper layer of the eye, which still retains its cup-like curve, 



m, 

 ri 



Fig. 164. — A, rudiment of the eye in a Hydrophilus larva just 

 hatched. B, a somewhat older larva (after Patten), cl, 

 chitinous lens ; h, hypodermis ; I, lentigen layer; m, middle 

 layer of the optic rudiment ; n, nerve ; o, aperture of the 

 optic invagination ; r, retinal layer ; rb, rods (in A arranged 

 in a single row). 



