208 COLLEGE ZOOLOGY 



does not often fall a prey to large enemies, but it is protected 

 from them by the withdrawing reaction, by its locomotion away 

 from the light, and by its habit of pulling eel grass and other 

 debris over the body." (Pearse.) 



Regeneration. -- Sea-cucumbers possess remarkable powers 

 of regeneration. When one is irritated it contracts the muscles 

 of the body- wall, and " since the fluid in the body-cavity is 

 practically incompressible, the effect is to set up a tremendous 

 pressure. As a result of this, the wall of the intestine near the 

 anus tears, and a portion or the whole of the intestine is pushed 

 out. The gill trees are the first to go, and in some species the 

 lower branches of these are covered with a substance which 

 swells up in sea-water into a mass of tough white threads in 

 which the enemies of the animal are entangled. A lobster has 

 been rendered perfectly helpless as a consequence of rashly in- 

 terfering with a sea-cucumber. These special branches are 

 termed Cuvierian organs. 



" A Holothurioid is only temporarily inconvenienced by the 

 loss of its internal organs. After a period of quiescence it is 

 again furnished with the intestine and its appendages. Some 

 species, which are able to pull in the mouth end of the body with 

 their tentacles, when strongly irritated snap off even this, and 

 yet are able to repair the loss." (Shipley and MacBride.) 



Economic Importance. - - Among the South Pacific islands 

 and on the coasts of Queensland and in southern China, dried 

 holothurians are known as " beche-de-mer " or " trepang " and 

 are used for food. The trade mounts into hundreds of thou- 

 sands of dollars annually. 



6. CLASS V. CRINOIDEA SEA-LILIES OR FEATHER-STARS 



Distinctive Characteristics. - - Attached by aboral apex of 

 body during early stages of development; arms usually branched 

 and bearing pinnules; tube-feet like tentacles, without ampullae. 



There are five or six hundred living representatives of this class; 

 fossil remains are very abundant in limestone formations. Most 



