110 ANIMAL BIOLOGY 



Oysters, clams, and other mollusks which are too large to 

 be taken into the body are often digested in this way, after 

 which the stomach retracts leaving the empty shells. 

 Starfishes often do much damage to oyster beds, and they 

 are very difficult to get rid of. 



Scattered over the body of many kinds of starfish are 

 numerous minute bodies called pedicellaricz which look 

 like miniature pairs of forceps. These organs have usu- 

 ally two jaws which open and close by means of special 

 muscles. They frequently catch 

 hold of objects coming in contact 

 with the starfish, and are thus 

 serviceable in capturing prey. A 

 live fish, longer than the diameter 

 of the starfish, has been observed 

 to be held by these minute organs 

 until it was conveyed by means of 

 the tube feet within reach of the 

 extensile stomach. 

 from below showing the The power of regeneration is 



mouth. 



very well developed in most star- 

 fishes. They will easily restore missing rays and even 

 considerable parts of the disk, and there are a few species 

 in which a single ray may give rise to an entire individual. 



The Ophiurans, or brittle stars, may be distinguished 

 from the starfishes by their circular disk which is clearly 

 set off from the slender arms. When an arm is seized 

 it is frequently cast off by a violent muscular contraction, 

 hence the term brittle star. 



In the Echinoids or sea urchins the body is generally 

 circular or oval in outline and covered with movable 

 spines. Generally also sea urchins are provided with tube 

 feet having adhesive terminal disks which are used much 

 as they are in the starfish. The spines are joined by a 



