

THE ECHINODERMS 



The last group of the Echinoderms, the sea-cucumbers, are 

 not well represented in the shore pools, but species of Cucumaria 

 do occur there. In C. lactea the body is about an inch long, and 

 the animal has a worm-like appearance which makes it very 

 different in appearance from its allies. The skin is tough, and 

 contains very much less lime than that of sea-urchins or starfish. 

 Down the cylindrical body run five rows of tube-feet, which are 

 short and much less freely movable than in the sea-urchins. 

 The mouth is at one end of the body and is surrounded by beau- 

 tiful branched tentacles, while the anus is at the other. A great 

 difficulty in keeping the sea-cucumber in an aquarium is that it 

 mutilates itself, especially by throwing out the internal organs, 

 upon such trifling provocation that it is very difficult to get a 

 perfect specimen to place in the aquarium. In the brittle-stars 

 the habit of throwing off the arms is as clearly protective in 

 function as is the lizard's habit of throwing off the tail when 

 this is seized. In both cases the separated part continues to move 

 actively, and distracts the attention of the enemy while the animal 

 makes its escape. It is, however, much more difficult to see the 

 value to the animal of the sea-cucumber's habit of throwing out 

 its internal organs on an alarm, even if these can be rapidly re- 

 grown. 



