20 STEREOTROPISM OF THE DOGFISH 



stimulus applied to the right upper surface of the snout would at one 

 moment cause the dorsal fins to turn to the right, while at another 

 moment a stimulation of the same region caused these fins to bend 

 to the left. The paired fins and the tail participated in these res- 

 ponses, and the direction of their movements had a definite 

 relation to the movements of the dorsal fins. It became apparent 

 that these fin movements were always consistent among themselves; 

 they were more than simple reflexes, and showed a coordinated 

 adjustment of the organism as a whole. In general they could be 

 seen to exhibit such an arrangement as would be necessary to turn 

 the animal either in the direction of the stimulating object or away 

 from it. That is to say, the reactions were in each case stereotropic, 

 but the sense of the stereotropism could be positive or negative. 

 It became then a matter of interest to determine, if possible, the 

 conditions of the reversal, and so to control these conditions as to 

 make the responses predictable. This proved to be indeed very 

 simple. 



METHODS. 



In making these experiments on the effects of contact stimuli it 

 would have been desirable to keep the fish in its natural position in 

 the water. This however was impracticable because the mechanical 

 effect of the stroke or push which constitutes the stimulus was suffi- 

 cient to move the body of the fish under the unstable conditions of 

 water support only. Moreover the stimulus excited movements of 

 locomotion and the observer was unable to keep track of the positions 

 and changes of positions of the different fins. If the aquarium used 

 was large the fish was soon out of reach; if small, new stimuli were 

 ofi'ered by collision with the walls. Another disturbing factor, if 

 the animal is floating in the water and free to move, is the fact that 

 each response to a tactile stimulus causes such a change of position 

 as to excite the labyrinth and thus introduce other reflexes. It was 

 necessary, therefore, to use the ordinary method of artificial respira- 

 tion by means of a current of aerated sea water through a rubber 

 tube in the animal's mouth. 



When the dogfish is first placed in the shark board rather violent 

 struggles occur, and tying is usually necessary until the animal be- 



