174 



COMPARATIVE ANATOMY 



CHAP. 



the cornea, within the optic cup or on the plate, lies a cellular lens, 

 which in the dorsal eyes of Oncidium consists of a few (5) large cells, 

 but in the pallial eyes of Pecten and Spondylus of very numerous cells. 



FIG. 150. Section through the eye of Pecten (after Patten), c, Cornea ; I, lens ; ep, pig- 

 mented body epithelium ; </, layer of ganglion cells ; r, retina ; st, rod layer of the retina ; d, tap- 

 etum ; e, pigmented epithelium ; /, sclerotica ; n, optic nerve ; n\ and 7i 2 , its two branches. 



The development of this lens is unknown ; it is perhaps formed by a 

 thickening or invagination of the embryonic ectoderm which covers 

 the eye. 



In Oncidium, the optic nerve penetrates the wall of the optic cup, as in the 

 vertebrate eye, to spread out on the inner surface (with regard to the centre of the 

 vesicle) of the retina, and to innervate the retinal cells. 



