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 givethe 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 tothe 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 thebody. It isobviousthat 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. 
