IX 



PHYLUM ECHINODERMATA 



423 



B 



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

 tree-like dermal branchia? 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 

 fenestra?. 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. Sphseridia are 

 present, but pedicellariaB 

 and clavulaB are absent. 



An " AvitntlA' lanfprn " FIG. 340. Hemipneustes radiatus. A, aboral, 

 An A11ST and ^ oral surfaco _ c> apical plates. (From Bromi's 



with teeth is present, as Tien-etch.) 



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- 



FIG. 34i.-ciypeasterj:sub- tiiiguishable : it is most distinctly de- 



depressus, view of aboral i T- ? i IV l T 



surface showing the petaloid velopecl in 1 solus and allied genera. 

 SSg' < FromHertwi e' E some Holothuroids the surface is enclosed 



