ECHINODERMATA 



185 



Fig. 166. — Diagram of a Very Young Asteroid 

 At the base of the 5 terminal tentacles is an optic cushion with a bright red 

 ocellus, Oc, connected by an epidermal radial nerve which runs to the central 

 nerve pentagon surrounding the mouth (after Lang). Compare Plate I. 



said to show the most rapid reactions (Scheer, 1956). In the Echinoid, Diadema, 

 the distribution of sensitivity corresponds to the distribution of the nerve 

 elements and it may be that these are directly stimulated bj- light as we have 

 seen to occur in the apolar light-sensitive cells of w- orms (Millott, 1954). On the 

 other hand, photosensitive pigments may be present in minute quantities, but 

 there is yet no evidence as to their nature. 



Many sea-urchins have the same primitive sensitivity associated particu- 

 larly with their pigmented spicules which move on the stimulus of light (v. 

 Uexkiill, 1900), and in some types characteristic iridescent bodies associated with 

 melanin pigment lie near the spines {Diadema antillarutti) (P. and F. Sarasin, 

 1887 ; Dahlgren, 1916; Millott, 1950-54). These represent clusters of regularly 

 arranged plates resembling iridoi^hores ^ in their arrangement, which presumably 

 act by reflecting the light onto the sensitive 

 spines (Millott, 1953) (Fig. 165). It is of historical 

 interest that the Sarasins (1887), in a much 

 quoted paper, described similar structures in 

 Diadema setosum, an allied species inhabiting 

 the Indian Ocean, as being "eyes" composed 

 of several hundred polygonal corneal facets, a 

 vitreous-like jelly and a '' retina", but without 

 nerve fibres. 



Cuf 



In STARFISHES (Asteroids such as the 

 common five-rayed Asterias), although the 

 skin is often diffusely light-sensitive, on 

 the tip of each of the five arms a visual 



1 Compare iridocytes, p. 89. 



Fig. 167. — The optic cushion of 

 the Asteroid 



Cut, cuticle ; CT, connective 

 tissue ; Ep, epithelium ; NN, 

 nerve-net ; P, pigment cells. 



Sea-cucumber, 

 Holothuria 



The sea-urchin, 

 Diadema 



