250 ZOOLOGY 



the intestine, like that of the Gephyrea, is full of sand — or en- 

 tangling food particles ; and in the latter case the tentacles are 

 then thrust into the mouth, which removes any nutritive particles. 

 The radial vessels pass through notches in the radial ossicles of 

 the pharyngeal calcareous ring, and run along the ambulacra 

 giving off tube-feet outside the bundles of muscle fibres. They 

 are absent in one group, and devoid of tube-feet in others. The 

 ampullae of the tube-feet are embedded in the circular muscle 

 layer in the Elasipoda, and in many of this group the stone 

 canal opens on the dorsal surface, and in others it lies in the 

 tissue of the integument ; in other subdivisions it is supported 

 by the mesentery, and the madreporic plate opens freely into 

 the coelom. It may or may not have calcareous walls ; the 

 fluid in this system contains numerous corpuscles. 



The vascular system consists of spaces in the connective 

 tissue not lined by an epithelium. There is a circular space 

 round the pharynx, just behind the water-vascular ring. This 



Fig. 152. — Sea Cucumber Ciuiumaria crocea (Falkland Islands) bearing its young. 

 After Sir Wyville Thomson and Murray, "Cliallenger" Narrative. 



communicates with a dorsal and a ventral intestinal vessel, and 

 these two are connected by numerous anastomoses round the 

 walls of the alimentary canal. The dorsal vessel is in connec- 

 tion with a plexus which surrounds the left respiratory tree. 

 There are no radial vessels. 



The circumoral nerve ring gives off five radial nerves, and 

 nerves to the tentacles. There is a nerve plexus in the skin 



