1 86 The Animal Mind 



The tendency shown by many aquatic animals to orient 

 themselves with head up-stream, and to swim against the cur- 

 rent, was formerly thought to be a response to the pressure 

 exerted by the current a reaction leading the animal to re- 

 sist pressure. Lyon, however, pointed out that this explana- 

 tion assumes rheotropism on the animal's part. It is because 

 the animal opposes the current that the current exerts any 

 pressure. If it merely allowed itself to be carried passively 

 along, and if the current surrounding the animal flowed with 

 uniform velocity in all its parts, no stimulus whatever could 

 be exerted by the water pressure (254). It seems probable 

 that eyeless animals do not, as a matter of fact, orient them- 

 selves against a current of this sort, and that rheotropism in 

 their case occurs when a current of unequal velocity disar- 

 ranges their movements, or when they are in contact with a 

 solid body. Thus Jennings has suggested that in Parame- 

 cium the reaction is due to the fact that unless the animal has 

 its head to the current, the flow of the latter will interfere 

 with the normal backward stroke of the cilia, causing negative 

 reactions until the disturbance is removed by proper orienta- 

 tion (211, p. 74). In animals with eyes, however, there is 

 reason to think that apparent rheotropism is largely an affair 

 of vision. Lyon's theory of rheotropism in fishes is that the 

 fish orients itself and swims in such a way that its surround- 

 ings, the bottom of the stream, for example, shall appear to the 

 sense of sight to be at rest, an hypothesis which, as we shall 

 see, was adopted by Rdl to explain the "hovering" of insects 

 in one place (355). Lyon supports it by experiments where 

 the bottom or sides of the aquarium were caused to move in 

 the absence of any current in the water, and the fish was 

 found to follow them. When the fish was placed in a re- 

 volving glass cylinder, it followed the revolutions, although 

 there was a slow current, of course, in the same direction, 



