206 MANUAL OF ZOOLOGY. 



their under surface excavated into ambulacral grooves. The 

 arms, in fact, are not simple prolongations of the body, as in 

 the Asteroidea^ but are special appendages, 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. The lateral plates 

 generally carry more or less well-developed spines. In the 

 centre of each arm is a chain of quadrate ossicles, forming 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 con- 

 tinued into an intestine, the mouth serving as an anal aperture. 

 The stomach is destitute of lateral diverticula. The repro- 

 ductive 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.* 



The ambulacral system is constructed upon the same plan 

 as in the Echinoids and Asteroids ; but its place as a loco- 

 motive apparatus is 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 placed on 

 the inferior surface of the body, and is often partially concealed 

 by one of the plates surrounding the mouth. 



Respiration is carried on by the lining of the body-cavity, 

 and by a circlet of modified tube-feet or tentacles placed round 

 the mouth. 



The development of the Ophiuroids is sometimes direct, 

 the young, being brought forth alive, and, in some cases, being 

 carried by the mother for some period after hatching (Wyville 

 Thomson). More commonly there is a pluteiform embryo, 

 which resembles that of the Echinoids in having a continuous 

 endoskeleton. 



In Euryale the body is in the form of a subglobose disc 

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

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



* Spontaneous fission has been observed by Ltitken and Kowalewsky 

 to take place in some Ophiuroids, as also occasionally in some of the 

 Asteroids. 



