34 TROPISMS 



square trough, two opposite sides of which were formed 

 of platinum electrodes. The cross section of the fresh 

 water in the trough was 1,400 mm. 2 and the intensity 

 of the current about 1 milliampere or a little less. We 

 found it advisable to increase the intensity very gradually 

 by increasing slowly the resistance of a rheostat in a 

 short circuit until the phenomenon of galvanotropism 

 appeared most strikingly. When the current is too strong 

 or too weak the phenomena are no longer clear. The com- 

 mon shrimp (Palcemonetes) is a marine crustacean which 



FIG. 2. Forced position of shrimp (Palcemonetes) when galvanic current goes from 

 head to tail. Tension of extensor muscles of tail fin prevails over that of flexors. 

 Animal can swim forward (to anode), but not backward. (After Loeb and Maxwell.) 



lives also in brackish water and which can stand fresh 

 water long enough for the purpose of these experiments. 

 The animal can swim forward as well as backward; in 

 forward swimming the extensor muscles of its tail fin 

 work more strongly than the flexors (Fig. 2) ; in swim- 

 ming backward the flexors work energetically (Fig. 3) and 

 thus produce a powerful stroke forward, while the ex- 

 tensors contract with less energy. When we put a Palce- 

 monetes in a trough through which a current goes and if 

 we put the animal with its head toward the anode the tail 

 is stretched out (Fig. 2). This means that the tension of 

 the extensor muscles prevails over that of the flexors 

 and since the forward swimming is due to the stroke of 



