STARFISH 



199 



228. Feeding and Metabolism. — As the starfish moves about over a 

 surface it secures animals which are unable to run away, such as oysters, 

 mussels, barnacles, clams, snails, and tube-dwelhng worms, or those 

 which are surprised and captured before they can escape, such as crus- 

 taceans and even small fish. When an object is captured which can be 

 passed through the mouth into the stomach, it is done and digestion 

 takes place within the body, after which the indigestible part is thrown 

 out through the mouth. If, however, a starfish finds itself over such an 

 animal as an oyster or mussel, which is firmly attached and protected 

 by a shell, it has recourse to a novel mode of circumventing its prey. 

 Tube feet are attached to the two valves of the shell and then a steady 

 pull is exerted which tends to draw the valves apart. This pull may be 



i:?^^ 



Fig. 113. — Asterias feeding on a clam. The valves of the clam are pulled apart by the 

 tube feet and the stomach of the starfish is then everted about the soft parts of the clam. 

 The stomach juices of the starfish may aid in causing the valves of the clams to open. 

 {From H. L. Wieman, "General Zoology" (1938), by the courtesy of McGraw-Hill Book 

 Company, Inc.) 



resisted by the mollusk for some time, but sooner or later the muscles of 

 the victim relax. The stomach of the starfish is then everted through 

 the mouth and immediately inserted into the crack between the two 

 valves. Though the mollusk may for a time again attempt to close 

 this crack, the starfish ultimately succeeds in inserting enough of its 

 stomach so that it can be wrapped about the body of the mollusk, which 

 is then digested within its own shell. Wlien digestion is complete the 

 starfish draws the stomach back into its body by means of retractor 

 muscles and moves on to find other prey. It is stated by MacBride 

 that if the mollusk is not firmly attached, the starfish will pick it up 

 between its rays; on one occasion a starfish which was confined was 

 observed to walk about all day carrying with it a mussel which it was 

 unable to open. 



