450 GEORGES BOHN 



are easily explained by invoking the conception of speed of 

 chemical reaction. 



Rose *" has studied tropisms to light and heat and the corres- 

 ponding differential sensibility in Daphnia. x\mong other results 

 he verified the observation of Loeb that weak (n/500) mineral 

 acids are sensibilisators. He verified also a law which I 

 have pointed out: any sudden variation in the light produces a 

 vertical displacement in the animals : if the intensity is suddenly 

 increased they sink, if it is diminished they rise. He recog- 

 nized that Weber's Law would be applicable in this connection, 

 and he has extended to warmth the results which have been found 

 for light. The notion of differential sensibility based on certain 

 laws has shown itself fertile here. 



Drzewina ^* has studied the reactions to light of Clihanarius 

 misanthropiis Risso. At Arcachon, on the Atlantic, she 

 noted a curious parallelism between the fortnightly movements 

 of the tide and the periodicity of variations in the sign of photo- 

 tropism. But at Banyuls, on the Mediterranean," she always 

 found marked positive phototropism, a fact which is not surpris- 

 ing in view of the different ethological conditions. However, 

 starting with the hypothesis that heliotropic sensibility is a 

 function of the speed of chemical reactions, D. succeeded 

 in modifying the sign of the reactions to light. If the water in 

 the vessels is daily changed, the phototropism remains positive, 

 but it becomes negative in unrenewed water. The addition of 

 sea salt (i to 100) raises the sensibilit}^ to light and neutralises 

 the influence of impure water. With carbonic acid (10 parts of 

 Seltzer water to 100) change in the sign of the phototropism 

 results at once. Individuals long exposed to light- and those 

 subjected to prolonged shocks finally move towards the shade. 



Further, Drzewina " has made an elegant application of the 

 associative method to the study of sensations in these crabs. 

 She put naked hermit crabs near shells hermetically closed with 

 cork; the animals at once fastened upon them, trying to tear off 

 the cork. Their efforts were in vain, and in time they appeared 

 more and more indifferent to the shells; six or eight days after 

 the beginning of the experiment, when they encountered them, 

 they continued on their course, or even pushed the shells away. 

 Now if at this point shells were introduced similarly stopped 

 with cork but of a different shape, the behavior of the animals 



