ANNULOIDA: ECHINODERMATA. 147 



considered as belonging to the Asteroidea, to which they are 

 nearly allied. The body in the Ophiuroidea (fig. 41) is dis- 

 coidal, and is covered with granules, spines, or scales, .but 

 pedicellariae are wanting. From the body which contains all 

 the viscera proceed long, slender arms, which may be simple 

 or branched, but which do not contain any prolongations from 

 the stomach, nor have their under surface excavated into am- 

 bulacral grooves. The arms, in fact, are not simple prolonga- 

 tions of the body, as in the Asteroidea, but are special appen- 

 dages, superadded for locomotive and prehensile purposes. 

 Each arm is enclosed by four rows of calcareous plates, one on 

 the dorsal surface, one on the ventral surface, and two lateral. 

 In the centre of each arm is a chain of quadrate ossicles, form- 

 ing a central axis, and between this axis and the row of ventral 

 plates is placed the ambulacral vessel. Each ossicle of the 

 central chain is composed of two symmetrical halves, but these 

 are immovably articulated together, and are not movable upon 

 one another, as in the Asteroidea. The mouth is situated in 

 the centre of the inferior surface of the body, is provided with 

 a masticatory apparatus, and is surrounded by tentacles. It 

 opens directly into a sac-like ciliated stomach, which is not 

 continued into an intestine, the mouth serving as an anal aper- 

 ture. The stomach is destitute of lateral diverticula. The 

 reproductive organs are 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 

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

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

 terophyton, the Medusa-head star, the arms are divided from 

 the base, first dichotomously, and then into many branches. 

 In Ophiura, the sand-star, the arms serve for reptation (creep- 



