no THE BIOLOGY OF THE SEA-SHORE 



Power of Contraction of Adductor Muscles of Common 

 Bivalves (after Plateau) 



Per sq. cm. — sectional area 

 Venus verrucosa .. .. .. 12,431 grams. 



Pectunculus glycimeris 



Mytilus edulis 



Ostrcsa hyppopus . . 



Ostrcea edulis 



Cardium edule 



Anodonta cygnea (fresh-water) 



10,152 



7.984 

 6,365 

 5,867 

 2,856 

 702 



For purposes of comparison the power of contraction 

 of the muscles of a man's calf, calculated on the same basis, 

 may be put at 9000-10,000 grams, that of frog muscle at 

 3000 grams. 



Reciprocal Adaptations on the Part of Foes. — Where 

 a particular form has some inveterate enemy it seems 

 that the struggle between them has favoured reciprocal 

 adaptations of attack and defence, which result in the 

 establishment of a sort of equilibrium in which the two foes 

 are about equally matched. A German worker, Bauer 

 (191 3), has made some interesting observations, though 

 many of his conclusions are of a distinctly speculative kind. 

 Bauer has observed Nassa reticulata escaping from pursuit 

 on the part of the starfish Astropecten bispinosus by a series 

 of curious leaps, the snail employing the foot as a lever in 

 the same way as the cockle. The movement is such a con- 

 vulsive one that it is difficult to follow but appears to be a 

 kind of somersault. The interesting feature, however, is 

 that the escaping reflex is only exhibited when the snail 

 comes into direct contact with the skin of the starfish, nor 

 is the snail's body everywhere equally sensitive to the 

 stimulus. It is only when the tentacular-like extensions of 

 the hind end of the body are touched that the reflex occurs. 

 The writer was unable to induce the reflex either mechani- 

 cally or chemically but only through the medium of a star- 

 fish, consequently one is forced to conclude that this is a 

 case of a very specialised kind in relation to a natural enemy. 



