1S4 



MANUAL OF ZOOLOGY. 



and open by orifices on the ventral surface of the body or in 

 the interbrachial areas. 



It is very questionable whether the ambulacral system in the 

 adult Ophiuroidea communicates with the exterior; its place 

 as a locomotive apparatus being taken by the arms. The 

 radial vessels of the ambulacral system are not provided with 

 secondary vesicles or " ampullae," as they are in the Echinoidea 

 and Asteroidea, and the lateral " feet" which they give off have 

 no terminal suckers. The madreporiform tubercle is either 



Fig. 43, — Ophiuroidea. a 0J>7tinyn texi-urata, the common Sand-star; 

 b Ophiocoma tteglecta^ the grey Brittle-star (after Forbes). 



placed on the inferior surface of the body or is partially con- 

 cealed by one of the plates surrounding the mouth. 



The larva of the Ophiuroidea is pluteiform, and is furnished 

 with a continuous endoskeleton ; and in some, as in Ophio- 

 lepis squamata, the echinoderm-body appears within the larva, 

 when the latter has attained but a very imperfect degree of 

 development. 



In Euryak the body is in the form of a sub-globose disc 

 with five obtuse angles, and the arms are prehensile. In As- 



