468 



COMPARATIVE ANATOMY 



CHAP. 



transformed spines, have already been described in the section on the 

 skeletal system (pp. 392-3). According to recent researches, there is 

 only a loose connection between the sphaeridium and the shell tubercle, 

 fibres of connective tissue, and not muscle fibres, uniting them. When 

 an Echinoid is in the natural position, the sphseridia, which are 

 developed only on the oral side, hang down perpendicularly in their 

 niches or chambers, owing to the weight of the dense calcareous mass 

 which forms their rounded ends. They are thus able, by pressing on 

 the nerve cushion at their base, to orientate an animal as to its position 

 in space. Erkinoids which are laid on the back, quickly turn them- 



selves over again. 



D. Eyes. 



1. The eye-spots of Asteroids have already been mentioned 

 (p. 463) in connection with the terminal tentacles. A vivid red eye- 



FIG. 377. Section through the optic cushion at the base of the terminal tentacle of an 

 Asteroid. 1, Cuticle of the optic cup ; 2, pigment cells ; 3, cuticle of the tentacle epithelium (4) ; 

 5, nerve layer below the surface of the same ; 6, cutis of connective tissue ; 7, epithelium of the 

 tentacle canal. 



spot is found at the base of each of these tentacles, on the side turned 

 to the mouth. On closer examination each eye-spot is found to break 

 up into a large number of single eyes, shaped like cups or hollow 

 cones. The tips of these conical cups are directed inward towards 

 the highly developed layer of nerve fibres below the surface of the 

 tentacle epithelium, while their cavities open outward (Fig. 377). The 

 wall of each optic cup is formed of pigment cells (with interspersed 

 unpigmented retinal cells). The cuticle of the tentacle epithelium 



