ZOOLOGY 
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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 
Fie. 152.—Sea Cucumber Cucumaria crocea (Falkland Islands) bearing its young. 
After Sir Wyville Thomson and Murray, ‘‘ Challenger’”’ 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 
