VIII 



ECHINODERMA T A SENSOR Y ORGA NS 



469 



also is continued into each cup. The portions of the cuticle which 

 belong to the different cells forming the wall of the cup are distinct 

 from one another, and have been described as rods. 



Living Asteroids carry the arm with its tip directed upwards, so 

 as to enable the eye-spot to function. 



2. In Diadema setosum (Eucckinoidea diadematoida), an animal 

 highly sensitive to light, the skin, which is as black as velvet, is 



FIG. 378. Part of a compound eye of Diadema setosum (after P. and F. Sarasin). 



p, Pigment cups. 



ornamented with numberless shiny blue spots ; these are gradually 

 lost on the oral side. 



Each of these blue spots, when its surface is examined with the 

 microscope, breaks up into a number of pentagonal or hexagonal 

 portions. The number of these varies, according to the size of the 

 spot, sometimes being many hundreds. Each is a refractive body, 

 which stands in a cup of black pigment (Fig. 378). The blue colour 



. 3 



FIG. 370. Section through the eye of Diadema setosum, diagrammatic (after P. and F. Sara- 

 sin). 1, Layer of ganglion cells; -2, "cornea"; 3, refractive body; 4, pigment cups; 5, nerve 

 layer, or plexus ; 6, fibres of connective tissue ; 7, collection of pigment below the nerve layer. 



of the spots, which are regarded as compound eyes, is due to inter- 

 ference. 



A section made through such an eye (Fig. 379) reveals : (1) that 

 the body epithelium, which is covered by a ciliated cuticle, spreads, 

 much thinned, over the whole eye (cornea) ; (2) that each "refractive 

 body " consists of a number of vesicular cells (modified epithelial 

 cells) ; (3) that a pigment cup (consisting of cells which are often 

 branched and star-like) surrounds the basal portion of each refractive 

 body ; (4) that the whole eye, with its numerous pigment cups, rests 



