202 GENERAL ZOOLOGY 



placed in stagnant water, will throw out almost all of its 

 internal organs (entire alimentary canal, most of the nervous 

 and water- vascular systems), in fact, everything except the 

 body wall and the respiratory tree. Certain tropical species 

 when taken in the hands will turn into a slippery mass of 

 slime which cannot be held. Many of these sea-cucumbers 

 can, however, if conditions become favorable, regenerate all 

 their missing parts in a short time. 



CLASS 5. CRINOIDEA 



The crinoids, or stone-lilies (Fig. 82, A), live for the most 

 part at considerable depths in the ocean. They have five 

 branched arms which are covered with little tentacle-like 

 tube-feet. All crinoids are attached by the aboral pole 

 when young, but some species break loose from the stalk 

 when mature and swim about by waving their arms. The 

 permanently stalked species often attain a great length, 

 some measuring thirty or forty feet. Stone-lilies flour- 

 ished in great numbers during past geological ages, and lime- 

 stone deposits are often filled with their remains, the most 

 common objects being little calcareous discs from the stalks. 



