ECHIXODERMS 145 



139. The organs of defense and repair of injury. As we 

 have seen, the body-wall of the echinoderms is provided 

 with a series of plates, often bearing spines which serve as 

 organs of defense, and to protect the internal organs. The 

 starfishes and sea-urchins also possess numerous modified 

 spines (pedicellarid) scattered over the surface of the body, 

 which have the form of miniature birds' beaks, fastened to 

 slender muscular threads. During life these jaws continu- 

 ally open and close, and it is said they clean the body of 

 debris that settles on it ; but on the other hand there are 

 several reasons for the belief that they also act as organs 

 of defense. Thus protected, the natural enemies of echino- 

 derms appear to be relatively few, and are confined chiefly 

 to some of the fishes whose teeth are especially modified 

 for crushing them. In this 

 way, and owing to the action 

 of the breakers, they suffer 

 frequent injury, but many 

 species exhibit to a remark- 

 able degree the ability to re- 

 generate lost parts. Experi- 

 ments show that if all the 

 arms of a starfish be separa- 

 ted from the disk the latter 

 will within two or three 

 months renew the arms ; and FlG - i.-sand-doiiar. a flat sea-urchin. 



. Natural size. 



a single arm with a part of 



the disk is able to renew the missing portions in about the 



same length of time. 



The brittle-stars, as their name indicates, are usually ex- 

 cessively delicate, often dropping all of their arms upon the 

 slightest provocation ; but here again the ability is present 

 to develop the lost portions. 



Sea-cucumbers resent rough treatment by vigorously 

 contracting their muscular walls and removing from the 

 body almost the entire digestive tract, the respiratory tree, 



