306 



ECHINODERMA 

 Order III. Clypeastroidea. 



Flattened echinoids with central mouth and teeth; anus in the posterior inter- 

 radius, sometimes marginal; five petaloid ambulacral areas. Clypeastcr (fig. 

 302), Echinarachnius* (sand dollar, fig. 306), Mellita,* with holes through the 

 test. 



Order IV. Spatangoidea. 



Bilateral flattened forms more or less heart-shaped; mouth and anus ex- 

 centric, 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. 291) are upon the lower surface. Warmer seas. Spatangus* 

 (fig. 307), Echinocardium, Brissus. 



a A 



FIG. 306. FIG. 307. 



FIG. 306. Oral (A) and aboral (B) surfaces of the sand dollar, Echinarachnius 

 par ma. a, anus; g, genital pores; /, ambulacral areas; m, madreporite; o, mouth. 



FIG. 307. Young Spatangus purpureus (after Agassiz), the spines removed, 

 oral surface. In front, the slit-like mouth; behind, the anus. The bivium without 

 tubercles. 



Class V. Holothuroidea. 



The sea cucumbers are most removed of any group from the typical 

 echinoderm appearance. At the first glance, except in Psolus, the skin 

 appears naked and the characteristic plates absent. Yet these are im- 

 bedded in the skin in the shape of plates, wheels, and anchors. The 

 integument is leathery and muscular, with longitudinal and circular fibres. 

 The saccular body gives these forms a worm-like appearance, strength- 

 ened 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 ambulacra 

 alone are locomotor, those of the bivium being tactile or wholly absent. 



The alimentary canal (fig. 308) (except in Synapta) is coiled in a 

 uniform manner, although many minor convolutions may obscure this. 



