228 



HISTOLOGY 



ture (Fig. 200). The epithelium on the mantle edge showed pigmented 

 areas. As pigment is so often associated with light perception, experi- 

 ments were tried, and it was found that the mollusk reacted to light on 

 these spots and on them only. 



Sections reveal a thickening of the epithelium due to the simple 

 lengthening of the columnar cells. Their distal ends have acquired a 

 rather heavy mass of black or dark brown pigment. Otherwise, no 

 specific rhabdome is to be seen. 



A feature of this spot which helps to decide that it is an eye is the 

 transparent thickening of the very slight cuticle which is formed in this 

 region, into a flat lens. This lens is arranged so as to rather weakly 

 concentrate the light rays on the most pigmented cells. A form without 

 this, cuticular lens would represent the very simplest eye. 



-I. 



FlG. 200. Vertical section of an eye-spot on the mantle edge of Solen vagina; I., lens. (After 

 BENJAMIN SHARP.) 



The eyes of some starfishes furnish still other examples of a very 

 primitive light-perceiving organ. These forms also show, among their 

 different species, a gradual succession of stages which may be considered 

 to indirectly represent phylogenetic steps in the development of the 

 echinoderm eye. A very simple form is to be seen in Astropecten 

 Mulleri (Fig. 201). In this animal the simple epithelium of the body 

 is modified, over a portion of the radial nerve called the eye cushion, so 

 that it consists of two kinds of cells. 



The first are the supporting cells (sup.c., Fig. 201) which, besides 

 acting as support for the surrounding tissues, are used to produce from 

 their distal ends the thin, double-layered, outer cuticle (cu.\ Proxi- 

 mally they rest on a basement membrane (b. m.) which separates them 

 from the underlying connective tissue and muscle. 



Placed between these supporting cells, either singly (rare) or in 

 groups of from two to five or more, as seen in the section, are the other 

 modified epithelial cells, the light-perceiving cells or visual cells (vis. c.}. 

 They are stouter but more delicate in texture, and the nucleus is placed 

 as a rule farther distally in the cell than was the case in the supporting 

 cell. The distal end does not rest against the cuticle, but projects as a 



