390 



ZOOLOGY 



SECT. 



eccentrically situated near the margin. The ambulacra are petaloid. 

 The genital and ocular plates are usually more or less fused 



together at their edges, 

 C and the genital apertures 

 are often not in the geni- 

 tal plates, but in the 

 corresponding ambulacral 

 zones. The spines are ex- 

 ceedingly fine and hair- 

 like. Sphaeridia are pres- 

 ent, but pedicellarias and 

 clavulae 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 direc- 

 tion of the axis joining 

 mouth with anus, which 

 are placed at opposite 

 extremities (anterior or 

 oral, and posterior or 

 anal) of the body. The 

 shape is sometimes com- 

 pletely cylindrical, some- 

 times five-sided; in many 

 there is more or less 

 dorso-ventral compres- 

 sion, 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 dis- 

 tinguishable : it is most distinctly de- 

 veloped in Psolus and allied genera. In 

 some Holothuroids the surface is en- 

 closed in an armour of close-fitting 

 plates; but in the vast majority the 

 body- wall is comparatively soft, being 

 strengthened 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 surface showing the petaioia. 



, ,. .., T - i ., .-iinliulacTa. (From Hertwig's 



bodies with which it comes in contact. /.,/,, /,*/,/.) 



FIG. 311. Hemipneustes radiatus. A, from 

 above. B, from below. C, apical plates. (From 

 Bromi's T/ticrreich.) 



