414 



ZOOLOGY 



SECT. 



sometimes notched at the edges or pierced by fenestrae. The mouth 

 is in the middle of the flat or concave oral surface, the anus eccen- 



, trically situated near the 

 margin. The ambulacra are 

 petaloid. The plates of the 

 -.:.- apical system are situated 

 about the centre of the 

 v \ * aboral surface, sometimes 

 surrounding a centro-dorsal 

 plate ; sometimes more or less 

 fused together. The spines are 

 exceedingly fine and hair-like, 

 those of the dorsal surface ciliated. 

 Sphseridia and pedicellariaB are 

 usually present, but clavulae are 

 absent. The dorsal podia are 

 flattened and respiratory. A n 

 " Aristotle's lantern " with teeth is 

 present, as in the globular forms, 

 but often much simplified. 



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 some- 

 times completely cylindrical, some- 

 times five-sided ; in many there 

 is more or less dorso- ventral com- 

 pression, and the dorsal and ventral 

 surfaces may differ greatly from one another. A flattened sole- 

 like ventral surface bearing the three rows of tube-feet of the 

 trivium is, as already stated, often distinguishable : it is most 

 distinctly developed in Psolus and allied 

 genera. In some Holothuroids the surface 

 is enclosed in an armour of close-fitting 

 plates ; but in the vast majority the body- 

 wall is comparatively soft, being strength- 

 ened merely by a great number of minute 

 ossicles of a variety of shapes. In Synapta 

 (Apoda) numerous minute anchor-like 

 spicules, each connected with a latticed 

 plate, project from the surface, and cause 

 the animal to adhere to soft bodies with 

 which it comes in contact. In the pelagic 

 Pelagothuria and in Rhabdomolgus, as Hertwig's Lehrbuch.) 





FIG. 345. Hemipneustes radiatus. 

 A, aboral, and B, oral surface. C, apical 

 plates. (From Bronn's Tierreich.) 



FIG. 346. Clypeaster sub- 

 depressus, view of aboral 

 surface showing the peta- 

 loid ambulacra. (From 



