124 TBOPISMS 



Kreidl. 270 A crustacean, Palcemon, loses its otoliths in 

 the process of moulting and the animal curiously enough 

 replaces them by picking up small grains of sand and put- 

 ting them into its ears. Kreidl kept such crustaceans in 

 jars free from sand but containing line particles of iron 

 which the crustaceans after moulting put into their ears. 

 He expected that a magnet would now influence the ani- 

 mals as powerfully as gravitation, and this was the case. 

 When, e.g., he brought a magnet from above and the right 

 near the animal the latter turned to the left and downward. 

 The animal, therefore, behaved as if changes of pressure 

 of the otolith upon the nerve endings determined its 

 geotropic orientation. 



The theory meets with two difficulties which, however, 

 are not insuperable. First, removal of all the otoliths 

 does not interfere with the normal orientation of the ani- 

 mal. This might find its explanation in the fact that the 

 eyes act as a substitute. Delage had shown that if the 

 otocysts are removed in crustaceans or cephalopods the 

 animals lose their normal orientation more easily when 

 they swim about excitedly than do normal animals. In 

 order to show the effects clearly, however, it was neces- 

 sary to blind the animals. Animals which were merely 

 blinded but had their otocysts did not show these disturb- 

 ances of equilibrium. 119 



The second difficulty is the fact that animals which 

 possess naturally no otoliths are yet able to show such 

 geotropic reactions, e.g., certain crustaceans like Gelasi- 

 mus and Platyoniclius. We may assume that the pres- 

 sure of liquids on the nerve endings may have a similar 

 effect as the pressure of the otoliths. 



The next question is, How does the pressure on a nerve 

 ending bring about changes in the tension of muscles? 



