IX 



PHYLUM ECHINODERMATA 



399 



parts, which are prolongations of the coelome (Fig. 338, cosl. can). 

 Two of these the subtentacular canals form a pair separated from 

 one another by a median septum underlying the ambulacral groove. 

 The other the cceliac canal runs between these and the sup- 

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

 canal communicate with one another at the extremity of each 

 pinnule. 



The enteric canal begins with a wide, funnel-shaped O3so- 

 phagus leading to a spacious stomach which gives off a number of 

 short, blunt diverticula and a pair of longer, narrower, " hepatic " 

 caeca, which are slightly branched at the ends. Distally the stomach 

 becomes contracted and opens 

 into a wide intestine, which winds 

 round the coelome, becoming 

 narrower where it passes upwards 

 to open on the exterior, the 

 terminal pa^t, or rectum, project- 

 ing as a tubular papilla on the 

 surface. In the living animal the 

 rectal tube is observed to undergo 

 frequent movements of contrac- 

 tion 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. pro- 

 cess of intestinal respiration. 



The ambulacral system con- 

 sists of a ring-vessel surrounding 

 the mouth, and a series of radial 

 vessels (Fig. 338, rad. amb.) which 

 run in the ambulacral grooves, 

 giving off branches to the pin- 

 nules. Connected with the radial 

 vessels and their branches are a 

 series of minute tubular ciliated appendages, the podia or so-called 

 tentacles (Fig. 339, tent.), which are homologous with the tube-feet 

 of the Starfishes and Sea-urchins, but are devoid of terminal 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 tubular diverticula, the 

 water-tubes, which are suspended within the ccelome, and open 

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

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

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

 considered as together representing the madreporic canal and its 

 openings in th^ Starfish and Sea-urchm. 



flcial (ambulacral) nervous system ; ax. ne. 

 axial nerve ; cod. can. sub-tentacular and 

 coe ii a c canals ; mus. muscles ; neur. ves. 

 radial sinus of the perihaemal system ; rad. 



amb radial ambu cral vessel g^g ofl 



branches to the tentacles. Between the 

 paired sub-tentacular and unpaired coehac 

 canals is the genital rachis. The small round 



"* * K the 



