?AC> 



ECmNODERMA. 



Order III. Clypeastroidea. 



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

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

 petaloid ambulaoral areas. Clypeaster (tropical), EahinaradDiius* (sand 

 dollar, flg. 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. ) are upon the lower surface. Warmer 

 seas. Sjiatangris * EcTdnocardi inn, Brissits. 



Class V. Holothuroidea. 



The sea cucumbers are most re- 

 Fio. 334. Young Spatangui^ pur- movecl of aiiv STOUT) from the typical 



;'MrcH« (after Agassiz), the spines ^ ^ i JX 



removed oral surface. In front, echinoderm appearance. 



the slit-like mouth ; taehmd, the , ^ 



At the first 



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



bercles. " . . -^ -^ 



characteristic plates absent. Yet these 

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

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



'^^ 









^^1^ 



Fig. 3.35.— Demial plates nf HolotUurians. ,1, Mtirintroclnis rinltii. (After Paniels- 

 sen.) U, T/iyoiic Iirinrcus; C, Siinaiyta yirardii (orig.). 



longitudinal and circular fibres. The saccular body gives these 

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

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

 other echinoderm 8 these move with tlio 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) 

 Ijccomcs ventral, the bivium dorsal, and in many the trivial a-mbu- 



