228 GENERAL ZOOLOGY 



the bottom and the free edges of the closed valves lie just 

 beneath the captor's mouth. Then the starfish takes a firm 

 hold on the bottom with the tube feet of the outer portion 

 of its arms and simultaneously applies the suckers of the 

 remaining tube feet to both valves of the mollusk. 



Then the struggle begins. The mollusk has long before 

 closed its valves tightly by means of its strong adductor 

 muscles, and the only way the starfish can get at the soft 

 parts is to force the valves open. This it does by a long- 

 continued, steady pull on the surface of the valves, with 

 two sets of tube feet drawing in opposite directions. The 

 mollusk has great momentary strength, but it seems to be 

 as difficult for it to keep up the strain as it is for a man to 

 hold out his arm several minutes at a time. A single tube 

 foot of the starfish is very weak, and the combined strength 

 of all its tube feet measures less than the momentary 

 strength of an oyster. Yet, even with the exertion of much 

 less than its full strength, the starfish can open the valves 

 in from fifteen to thirty minutes. 



As soon as the valves are open a few millimeters the star- 

 fish contracts certain muscles in its body cavity, which 

 bring the stomach down toward the oral surface and cause 

 it to pass through the mouth opening, turning inside out 

 on its way. The everted stomach is applied to the soft body 

 of the mollusk, and digestion and absorption begin imme- 

 diately. With slight variations this is the method which 

 starfishes employ in opening and digesting oysters, snails, 

 and other mollusks. 



Starfishes are one of the most serious enemies of the 

 oyster industry. Oyster fishermen drag mop-like tangles 

 over the oyster beds. Starfishes become entangled in the 

 fibers and are then brought to the surface and destroyed. 



The Circulatory, Respiratory, and Excretory Systems. The 

 circulatory system is too small to be indicated in drawings 



