436 COMPARATIVE ANATOMY CHAP. 



4. Ophiuroidea (Fig. 367). In the Ophiuroidea, the ambulacral 

 appendages have no locoraotory significance ; they resemble tentacles, 

 and never have suckers. Locomotion is caused by the jointed arms 

 themselves. The tentacles are always strictly segmentally arranged, i.e. 

 one pair of tentacles occurs on each brachial segment. Each of these 

 emerges through an aperture between the ventral shield and the lateral 

 shield of a segment. The tentacles are not infrequently covered with 

 a large number of sensory papilla. A nerve, coming from the basal 

 circular ganglion and running in the deeper portion of the layer of 

 connective tissue, traverses the tentacle from base to tip. 



It has already been mentioned above that the first ten pairs of 

 tentacles (i.e. the first two pairs of each arm) have shifted, as oral 

 tentacles, to a position round the mouth, and receive their canals direct 

 from the circular canal. 



5. Crinoidea. The small tentacles which form the ambulacral 

 appendages of this class have already been sufficiently noticed (p. 414). 

 They never possess suckers, and have no locomotory function, but 

 simply serve for respiration, and for conducting food to the mouth. 



VII. The Ccelom. 

 (The Enterocoel. the true or secondary Body Cavity.) 



All those cavities of the body which are derived from the entero- 

 eoelomie vesicles of the larva are considered to belong to the coelom, 

 which is lined throughout with endothelium, usually developed as 

 ciliated epithelium. The ecelomie fluid exactly resembles in constitu- 

 tion the water vascular fluid already described. The ccelom is, how- 

 ever, except at one single point to be mentioned later, altogether 

 separate from the ambulacral vascular system. 



The coelom is never found as a single cavity, but is always divided 

 into several cavities, which may be entirely distinct one from the 

 other. The largest of these cavities is that which contains the viscera, 

 and which may be termed simply the body cavity. 



The body cavity is most spacious in the Echinoidea and the Holothu- 

 rioidea ; in these forms it occupies almost the whole cavity of the test, 

 or the sac- or tube-shaped body. In the disc of the Asteroidea it is 

 somewhat less spacious, and is very limited in that of the Ophiuroidea. 

 In the Crinoidea, it is traversed by a more or less strongly calcified 

 network of connective tissue. 



Where the body is drawn out into arms in the radii, the body 

 cavity runs into these, and forms the braehial cavities. The brachial 

 cavities in the Asteroidea are very spacious, but are much narrowed in 

 the Ophiuroidea and the Crinoidea, owing to the great development of 

 skeletal plates (vertebral ossicles, joints) in the arms. 



A special section of the ccelom, the pericesophageal sinus (peri- 

 pharyngeal sinus) encircles the oesophagus or pharynx. In the Echi- 



