242 GENERAL ZOOLOGY 



in the direction of the mussel and arches itself over the shell, 

 touching the bottom all around with the tips of its arms. 

 Next it turns the mussel about, until the hinge-ligament rests 

 on 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 mussel. 



Then the struggle begins. The mussel has long before closed 

 its valves tightly by means of its tAvo 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 mussel has great momen- 

 tary 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 a mussel. But the star- 

 fish can exert much less than its full strength, and open the 

 valves of a mussel within from fifteen to thirty minutes. 



As soon as the valves of the mussel are open a few milli- 

 meters the starfish 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 mussel, and digestion and absorption begin im- 

 mediately. With slight variations this is the method which 

 starfishes employ in opening and digesting oysters, snails, and 

 other mollusks. 



The Circulatory, Respiratory, and Excretory Systems. The 

 circulatory system is too small to be indicated in drawings 

 on the scale of the dissection drawings. A circular blood-vessel 

 4ies just below the ring-canal of the water-vascular system, 



