RHEOTAXIS 37 



RHEOTAXIS 



Under the head of rheotaxis are included the movements 

 of animals as directed by currents. This response is shown 

 even among very low forms. The plasmodium of the slime 

 mould Aethalium when placed on a filter paper along which 

 a current of water is passing slowly creeps opposite the 

 direction of the flow. It is not improbable that spermatozoa 

 migrate up the oviduct on account of the current which 

 the beating of the cilia causes to flow toward the uterus. 



Among fishes orientation to currents is a common phenom- 

 enon. Many fishes have the instinct to head up stream 

 and swim against the current. Lyon has shown that this 

 is due in large measure to reactions to movements of objects 

 in the visual field. In his experiments he used an aquarium 

 with a glass bottom "so supported that the bottom was 

 freely accessible. Close along the bottom, beneath the glass, 

 could be drawn a long piece of white cloth with black 

 strips painted across it. This would give the impression of a 

 moving bottom. Fish (Fundulus) placed in the aquarium 

 oriented themselves with the head in the direction of the 

 moving bottom and swam along it to the end of the aquarium. 

 Reversing the movement of the bottom reversed the orien- 

 tation and movement of the fish." 



The orientation of animals to currents has been explained 

 as due to differences in pressure produced by the current in 

 different parts of the body. It is obvious that such differences 

 can be produced only when the current moves past the animal. 

 Where the animal is entirely immersed and is carried along 

 passively the effect, as Lyon points out, is the same as if 

 it were in quiet water. It is like a man in a balloon carried 

 by the wind. Moving at the same rate as the air about him 

 he becomes conscious of motion only when he can see objects 

 on the earth passing by beneath. 



