No. 19.] -ECHINODERMS OF CONNECTICUT. 3I 



But experiments show that a mollusk taken away from a star- 

 fish which had opened it, soon regains its normal condition. 

 Another theory was that the starfish wrapped its stomach about 

 the mollusk's shell, depriving the animal of oxygen and eflFec- 

 tually smothering it. But observation shows that such is not 

 the case. Another theory that has been held is that an acid 

 dissolves the edges of the shell, but no such acid is present. 

 Others have believed that the edges of the shell are chipped 

 sufficiently to allow the entrance of the starfish's stomach; and 

 it is true that the delicate edges are often accidentally broken by 

 the starfish, but such a shell can still be closed so tightly as to 

 hold water. 



Thus the problem remained unsolved until 1896, when the 

 experiments of Schiemenz* showed that simply the long con- 

 tinued pull of the starfish's arms will eventually overcome the 

 resistance of the adductor muscle which holds the two valves 

 of the shell together. While the muscle can resist for a short 

 time a very strong pull tending to separate the valves, the muscle 

 eventually becomes fatigued, and relaxation allows the valves 

 to gape. In the same way, if a sufficiently stout cord is tied 

 around the foot of a large marine snail, a strong man is unable 

 to pull it from the shell. Yet such a snail is unable to support 

 its own weight, by the same muscles, for a long period. After 

 being suspended for a time by the foot, the muscles become 

 fatigued and relax, allowing the body and shell to hang loosely 

 at the end of the fully extended foot. 



Schiemenz demonstrated that the starfish was able to exert 

 a pull of more than 1,300 grams, while it required a force of 

 only 900 grams to fatigue the muscles of a large bivalve so 

 that the shell would gape widely within a half-hour. 



It is only necessary therefore for the starfish to grasp the 

 oyster with the tube-feet of opposite arms attached to the valves, 

 and then continue pulling in opposite directions until the mol- 

 lusk's adductor muscle becomes sufficiently -fatigued to allow the 

 valve to open. But a considerable force is required in order 

 to accomplish this, and it is only when the arms of the star- 

 fish are wrapped around the shell of the bivalve in such a 

 manner that the tube-feet in the basal portions of the arms can 



* Mitteilungen des deutschen Seefischervereins, 1896. 



