PHYLUM ECHINODERMATA 



423 



neighbourhood of the mouth, and sphaeridia also occur. A series of 



tree-like dermal branchia3 surround the peristome. The " lantern 



of Aristotle," with its teeth, 



is not represented. 



In the Clypeastridea or 



Cake-urchins the whole 



corona (Fig. 341) is usually 



greatly compressed so as 



to assume the form of a 



disc, sometimes notched at 



the edges or pierced by 



fenestrae. The mouth is 



in the middle of the flat 



or concave oral surface, the 



anus eccentrically situated 



near the margin. The am- 

 bulacra are petaloid. The 



genital and ocular plates 



are usually more or less 



fused together at their 



edges, and the genital 



apertures are often not in 



the genital plates, but in 



the corresponding ambu- 



lacral zones. The spines 



are exceedingly fine and 



hair-like. Sphasridia are 



present, but pedicellariaa 



and clavulse are absent. 



An " Aristotle's lantern " 



with teeth is present, as 



in the globular forms. 



In the Holothuroidea the body is more or less elongated in 



the direction of the axis joining mouth with anus, which are placed 

 at opposite (anterior or oral, and posterior 

 aboral or anal) extremities of the body. 

 The shape is sometimes completely cylin- 

 drical, sometimes five-sided ; in many 

 there is more or less dorso- ventral com- 

 pression, and the dorsal and ventral sur- 

 faces may differ greatly from one another. 

 A flattened sole-like ventral surface bear- 

 ing the three rows of tube-feet of the 

 trivium is, as already stated, often dis- 

 tinguishable : it is most distinctly de- 

 veloped in Psoliis and allied genera. In 

 some Holothuroids the surface is enclosed 



FIG. 340. Hemipneustes radiatus. A, aboral, 

 and B, oral surface. C, apical plates. (From Broim's 

 Tierreich.) 



, ,341. ciypeasteivsub- 



depressus, view of aboral 



urface showing the petaloid 

 < FromHwtwi *' 8 



