IX 



PHYLUM ECHINODERMATA 



375 



other the cceliac canal runs between these and the supporting 

 ossicles (oss.). The sub-tentacular canals and the cceliac canal 

 communicate with one another at the extremity of the arms. 



The enteric canal begins with a wide funnel-shaped oeso- 

 phagus leading to a spacious stomach having a small ccccuni con- 

 nected with it. Distally it becomes contracted and opens into 

 a wide intestine, which winds round the coelome, giving off . at its 

 gastric end a number of small cascal diverticula, and becoming- 

 narrower where it passes upwards to open on the exterior, the 

 terminal part, or rectum, projecting as a tubular papilla on the 

 surface. In the living animal the rectal tube is observed to 

 undergo frequent movements of contraction and dilatation by 

 means of which water is 

 drawn into and expelled from 

 the intestine, so that here, as 

 in the Sea-urchin, there 

 would appear to be a process 

 of intestinal respiration. 



The ambulacral system 

 consists of a ring-vessel sur- 

 rounding the mouth, and a 

 series of radial vessels (Fig. 

 303, rad. ami.) which run in 

 the ambulacral grooves, giving 

 off branches to the pinnules. 

 Connected with the radial 

 vessels and their branches are 

 a series of minute tubular 

 appendages, the so-called ten- 

 tacles (Fig. 304, tent.), which 

 are homologous with the tube- 

 feet of the Star-fishes and Sea- 

 urchins, but are devoid of ter- 

 minal suckers. These are not 



organs of locomotion, they bear numerous sensory papillae, and are 

 therefore to be looked upon as tactile organs ; but they probably 

 also have a respiratory function. Connected with the ring- vessel 

 are a number of ciliated, branched, tubular diverticula, the water- 

 tubes, which are suspended within the coelome, and may open freely 

 into it at their extremities. A large number of vessels with 

 minute ciliated openings the water -pores (wat. p.) lead through 

 the ventral wall of the disc : these and the ciliated tubes are to be 

 looked upon as together representing the madreporic canal and its 

 openings in the Star-fish and Sea-urchin. 



The nervous system consists of two perfectly distinct parts. 

 A nerve-ring (ect. ne.) surrounds the mouth, and from it are given 

 off a series of amlulacral nerves thickenings of the epidermis of 



OX.TI& 



rod. amb 

 coel can. 



mus 



oss 



Fi. 303. An ted on, transverse section of a 

 pinnule, nm/j. ,n , anibulacral nerve; ax. n <;. 

 axial nerve: ccel. fin, sub-tentacular and 

 cceliac canals ; mug. muscles ; new. <<. blood 

 vessel : i-inl. ami:, radial ambulacral vessel. 

 (After Tcuscher.) 



