VISUAL TISSUES 



243 



The lens, as in all worms, is an extra-cellular material secreted largely 

 by the upper epithelium of the sac. In many other worms the lens is 

 secreted by a single special cell which lies just outside the retina and 

 passes the secretion into the optic vesicle through a cleft in the sensory 

 epithelium. This occurs on the extreme inner wall, in the worm Phyl- 

 lodoce ganimosa, the imagination of whose eye is not entirely completed. 

 It also is seen, in a somewhat peculiar form, in the eye of Vanadisformosa 

 as described by Hesse. We shall outline the structure of this eye as 

 a most highly specialized type of worm eye (Fig. 216). 



vit. 



FIG. 216. Eye of the worm, Vanadis formosa. I., lens; vit., vitreous body; vit.c., large vit- 

 reous cell which forms the vitreous body; ret., retina with narrow layer of pigment cells ; 

 nv.c., nerve cells forming ganglia. (From HALLER after HESSE.) 



The original invagination was the same as in Nereis, and the subsequent 

 formation of a cornea by the surface was also the same, excepting that a 

 small amount of connective tissue crept in between the cornea and the 

 optic sac, as we shall call the invagination. The difference comes in 

 the larger number and greater complexity of the structures formed by 

 the sac, and, what is still more significant, the addition of nervous ele- 

 ments to the rear of the retina, probably to correlate the image elements 

 and prepare them for their reception in the central ganglia. 



The retina consists, as in the other worms, of the specialized cells on 

 the proximal side of the optic sac. Unlike Nereis, however, this special- 

 ization is confined to a sharply marked area, and the visual cells are far 

 more numerous and larger. The pigment cells are reduced to a narrow 



