VISUAL TISSUES 



22 9 



rounded end into the space between the cuticle and a parallel inner 

 membrane called the limiting membrane. This space is filled with a 

 fluid in life, and the ends of the sensory cells which pass into it through 

 the limiting membrane are the cell-organs of light perception or visual 

 rods. 



The proximal ends of the visual cells do not reach to the basement 

 membrane, but are prolonged into delicate nerve fibers to conduct the 

 impulses to the nerve centers. These nerve fibers are seen in the draw- 

 ings as sections of numerous fibers which lie among the base of the tall 

 and thin supporting cells. 



FIG. 201. Parts of retinal tissue from the eyes of (.4), Astropecten MiUleri and (J5), Aslerias 

 tennispina; vis.c., visual cells; sup.c., supporting cells; /., lens; CM., cuticle; b.m., basement 

 membrane ; l.m., limiting membrane ; nv.f., nerve fiber layer. (After PFEFFER.) x about 600. 



This eye has no lens and is of extreme simplicity. Drawn in the 

 same figure with it is a representation of an eye from another starfish, 

 Asterias tennispina. On the optic cushion of this echinoderm the 

 visual cells are not distributed diffusely as in the preceding example, 

 but are collected into groups to form more definite " eyes." Each of 

 these groups is depressed into a cup-like hollow, carrying the limiting 

 membrane with it and displacing to some degree the surrounding sup- 

 porting cells. Such of these latter as immediately surround the depres- 

 sion bend over and touch the inner surface of the cuticle, and, besides 

 forming and supporting this cuticle, they deposit on its inner surface 

 a delicate, flat lens (I.). The lumen of this depression or invagination 

 remains open and is filled, in life, with a fluid. The sensory cells form 



