346 



ECHINODERMA. 



Order III. Clypeastroidea. 



Irregular flattened echinoids with central mouth and teeth ; anus out- 

 side the periproct in the posterior interradius, sometimes marginal ; five 

 petaloid ambulacral areas. Clypeaster (tropical), Echinarachnius* (sand 

 dollar, fig. 333), Mellita* with holes through the test. 



Order IV. Spatangoidea. 



Bilateral flattened forms more or less 

 heart-shaped ; mouth and anus excentric, 

 no teeth; usually five petaloid ambulacral 

 areas and four genital plates. From the 

 forward position of the mouth it follows 

 that only two ambulacral areas (bivium, 

 p. 334) are upon the lower surface. Warmer 

 seas. Spatangus* Echinocardium, Brissus. 



Class V. Holothuroidea. 



The sea cucumbers are most re- 



moved of an y s rou p from the to 1 



echinoderm appearance. At the first 

 anus. The bivium without tu- glance the skin appears naked and the 



characteristic plates absent. Yet these 



are imbedded in the skin in the shape of plates, wheels, and anchors 

 (fig. 335). The integment is tough, leathery, and muscular, with 



FIG. 335. Dermal plates of Holothurians. A, Myriotrochus rinkii. (After Daniels- 

 sen.) _B, Thyone briareus ; C, Synapta girardii (orig.). 



longitudinal and circular fibres. The saccular body gives these 

 forms a worm-like appearance, strengthened by its elongation in 

 the main axis, and with the mouth and anus at the poles. Unlike 

 other echinoderms these move with the main axis parallel to the 

 ground, a condition which, to a greater or less extent, leads to a 

 replacement of radial by bilateral symmetry. One surface (trivium) 

 becomes ventral, the bivium dorsal, and in many the trivial ambu- 



