268 



PRINCIPLES OF ANIMAL BIOLOGY 



arms. The sea urchins (C) are globular and without arms. Sea cucum- 

 bers {E) have no arms, but around the mouth is a series of branched 

 tentacles. The arms of the feather stars are branched like a feather, and 

 the branches are featherlike. 



The starfishes have the peculiarity of digesting their food outside the 

 body. They prey upon clams, forcing the valves of the shell open by a 

 steady pull with the tube feet. The stomach is thrust out through the 

 mouth, pushed between the separated valves, and wrapped around the 

 exposed parts of the clam, which is then slowly digested. Oyster beds 

 suffer considerably from these attacks. The other kinds of echinoderms 

 take their food inside the body. 



A curious habit is that of the sea cucumbers, of eviscerating them- 

 selves when irritated. If they are attacked, the body wall contracts so 

 vigorously that it bursts, and a part (or even all) of the intestine is forced 



A B C D E 



Fig. 225. — Various echinoderms: left to right, starfish, brittle star, sea urchin, sand dollar, 

 sea cucumber. {Courtesy of Carolina Biological Supply Co.) 



out, along with the branching respiratory organs that are attached to 

 the cloaca. The tangled mass of viscera may so hinder (or perhaps 

 appease) the enemy as to stop the attack. During a brief resting period 

 the missing internal organs of the sea cucumber are regenerated. 



Echinoderms are invaluable subjects in experimental laboratories 

 because of the abundance of their eggs and the ease with which they may 

 be obtained. Hundreds of studies of cytology, physiology of fertiliza- 

 tion, and embryology have been made on the eggs of starfishes and sea 

 urchins, and sometimes the other groups of echinoderms. 



The relationships of echinoderms to the other phyla have been much 

 debated because there is little clear evidence of them. Adult anatomy 

 is entirely different from that of any other animals, and conclusions 

 drawn from developmental stages have been various. There is less basis 

 for establishing kinships of echinoderms than of almost any other group. 



Class I. Asteroidea. Free-livinfi;, typically peiitamerous echinodoniis with wide 

 arms moderately marked off from disk and with ambulacral grooves. Starfishes. 

 (Fig. 2251.) 



Class II. Ophiuroidea. Free-living, typically pentameroiis echinoderms with 

 slender arms sharply marked off from disk and no ambulacral grooves. Brittle stars. 

 (Fig. 225B.) 



