286 



PHYLUM ECHINODERMATA. 



water- vascular trunk and the ambulacral groove which is sup- 

 posed to represent the perihaemal space of other types. It 

 appears to end blindly at each end, and there is no circumoral 

 representative of it. 



The water-vascular system consists of a circular vessel round 

 the mouth giving off a radial vessel into each arm. The radial 



FIG. 197. Diagram of a transverse section through the arm of a Crinoid (from Lang). 1 

 Radial nerve of the superficial oral system, 2 perihaemal canal (?), 3 radial water-vascular 

 trunk, 4 the radial trunks of the deep oral nervous system, 5 and 11 the ventral or 

 subtentacular canals, 6 the canal round the generative rachis, 7 the dorsal or coeliac canal, 

 8 radial trunk of the apical nervous system, 9 peripheral cutaneous termination of nerves 

 of 8, 10 connecting nerve between 8 and 4, 11 subtentacular canal, 12 nerve to tube-foot, 

 13 water-vascular canal of tube-foot, 14 sensory process on tube-foot, 15 ambulacral 

 groove. 



vessels branch with the arms and extend into the pinnules. 

 They terminate short of the end of the arms and there are no 

 terminal tentacles. The water-vascular trunks have muscular 

 walls and are sometimes traversed by muscular fibres. The 

 lining epithelium is not, as in other Echinoderms, ciliated. The 

 circular vessel gives off tubes which pass into the circumoral 

 tentacles, and a number of other tubes which hang down and 



