THE STARFISH AND SOME ALLIES 251 



mouth-opening. In captivity they appear to do this when 

 placed in unfavorable situations. It is not so remarkable that 

 they should lose their internal organs, as it is that they should 

 regenerate them all again as perfect as ever. If sea-cucum- 

 bers throw out their internal organs in nature, it would 

 appear to be a very expensive process. Since they do per- 

 form this action, in nature as well as in captivity, there must 

 be some reason for it. The only reason that suggests itself is 

 based on the fact that fish prey upon sea-cucumbers. The 

 sea-cucumber is a slow-moving animal, and once seen by a 

 roving fish the chances of escape under ordinary circumstances 

 would be few indeed. If the prospective victim when dis- 

 turbed were to eject its internal organs into the water, the 

 fish, true to the instinct of its kind, would gobble up the 

 swiftly moving object, and probably go away satisfied. 



Sea-cucumbers are prepared and used as food by the Chinese, 

 and it is reported that the Siwash Indians of the northwest 

 of the United States eat them, and sea-urchins also. 



DEFINITION OF ECHINODERMA 



Each of the five animals described in this chapter represents 

 one of the five classes that make up the phylum Echino- 

 der'ma (Gr. ecliinos, hedgehog ; derma, skin). The five classes 

 represented are Asteroi'dea, by Asterias; Ophiuroi' dea, by 

 Astrophyton; Crinoi'dea, by Pentacrinus ; Uchinoi'dea, by 

 Strongylocentrotus ; and Holothuroi'dea, by Cucumaria. 



The most important characteristics of the phylum are the 

 radial symmetry of the body (the importance of this character 

 is denied by some zoologists); the occurrence of repeated 

 divisions or organs of the body to the number of five, or 

 in multiples of five ; and the existence of a water-vascular 

 system. No other phylum has parts occurring in fives, and 

 none other has a water-vascular system. 



