ANNULOIDA I ECHINODERMATA. 



169 



situated near the bases of the arms, 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. 61. Ophiuroidea. a Ophiura texturata, the common Sand-star ; 

 b Ophiocoma neglecta, 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 Ophiolepis 

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

 the latter has attained but a very imperfect degree of develop- 

 ment. 



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



