ECHINODERMS 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 (pedicellaria) 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 
débris 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 
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, 
Fie. 91.—Sand-dollar, a flat sea-urchin. 
Natural size. 
