20 6 COLLEGE ZOOLOGY 



blindly near the anus {16), and tube-feet (Fig. 146). The circular 

 canal (2) gives off a polian vesicle (4) and one or more stone- 

 canals ending in internal madreporites {25). From ten to thirty 

 of the tube-feet surrounding the mouth are modified as tentacles 

 for procuring food. 



The alimentary canal includes a long looped intestine (Fig. 147, 

 23, 8, 22), the posterior end of which is a muscular enlargement 

 called the cloaca (15). Water flows into the cloaca through the 

 anus (16) and passes into two long branching tubes, the respira- 

 tory trees (11, 19) ; here part of it probably finds its way through 



Fig. 146. — A sea-cucumber, Thyone briareus, partly buried in mud. 

 (From Pearse in Biol. Bui.) 



the walls into the body-cavity. Respiration is carried on by the 

 cloaca, respiratory trees, tentacles, tube-feet, and body-wall. 

 The cloaca and respiratory trees also function as excretory 

 organs. 



Nutrition. — The food of most sea-cucumbers consists of 

 organic particles extracted from the sand or mud which is taken 

 into the alimentary canal. Some species are said to stretch out 

 their seaweed-like tentacles on which many small organisms 

 come to rest. " When one tentacle has got a sufficient freight 

 it is bent round and pushed into the mouth, which is closed 

 on it. It is then forcibly drawn out through the closed lips 

 so that all the living cargo is swept off." (Shipley and 

 MacBride.) 



