HOW DO STAEFISHES OPEN OYSTERS ? 



281 



exactly the strength of the starfish, but is considerably less. Indeed, 

 I have observed that if one endeavours to pull away again a mollusc 

 which has been offered it, a starfish will resist for some time, but if 

 the pull lasts too long, or is too strong, it quite suddenly draws in 

 all its tube-feet, lets the mollusc go, and cannot be persuaded to take 

 it a second time. However, if we accept 1350 grams provisionally as 

 representing the strength of the starfish, we shall, in what follows, be 

 able to show that so much power is far from being necessary for the 

 forcible opening of a moderate-sized Venus. 



This sounds unlikely, especially since one knows that, according 

 to Lawrence-Hamilton,* a Venus can, with its adductor muscles, with- 

 stand a strain equal to 2071 times its own weight (without the shell). 

 I have myself seen that, with a momentary weight of 4000 grams, a 



a, r : ■/ 



Fig. 9. 



Venus does not think of opening. But the circumstances are com- 

 pletely changed when, instead of a momentary strain, a continuous 

 one is applied. Everyone knows from his own experience that to 

 lift a weight, and to support it for a long time, are two quite different 

 things. 



In order to investigate the resistance offered by Vemts to a strain, 

 I had a jar of sea water sent from Naples, followed, some days after- 

 wards, by a number of Venus verrucosa (as samples without value) 

 wrapped in moist linen. The latter arrived in Hanover in three days, 

 and were in full vigour, protruding their siphons normally as soon 

 as they were placed in their native element. 



In order to measure their strength, I constructed, with the modest 

 appliances at my disposal, the apparatus figured above. (Fig. 9.) The 



* Lawrence-Hamiltux, J., "The Limpet's Strength." Nature, vol. xlv. p. 1S7. 1S92. 



