74 BEHAVIOR OF THE LOWER ORGANISMS 



end directed up stream. The reaction then ceases, and the infusorian 

 remains in this position, either swimming forward against the current, 

 or at rest against the wall of the tube. Sometimes the reaction is a 

 little more precise, the animal turning directly toward the aboral side 

 till the anterior end is directed up stream. This is commonly the 

 case with the individuals that are at rest against a solid. The reaction 

 of the resting specimens is less easily observed, for the current easily 

 carries them away from their attachment, when of course they behave 

 like other free specimens. 



What is the cause of the reaction to water currents ? Under natural 

 conditions the cilia of Paramecium are beating backward, driving a cur- 

 rent of water backward over the surface, especially in the oral groove. 

 If an external current moves in the opposite direction, or in some oblique 

 direction, it vdll of course act in opposition to the ciha on that part of 

 the body which it strikes, tending to reverse or disarrange them, and to 

 reverse or change the direction of the usual currents. It appears not 

 surprising that such a disturbance acts as a stimulus, causing the usual 

 avoiding reaction until the disturbance is corrected. The correction can 

 occur only when the animal is headed up stream; the current is then 

 passing backward over the body in the usual direction. The reaction 

 is essentially a response to a mechanical disturbance, comparable to that 

 due to the touch of a sohd body. 



If this is the correct explanation, as seems probable, then there should 

 be no reaction when the animal is completely immersed in a homogene- 

 ous current, — one moving at the same velocity in all parts. For as 

 Lyon (1904) has pointed out, under these circumstances the animal is 

 merely transferred bodily in a certain direction, along with the medium 

 surrounding it, and at the same rate. Its relation to the enveloping fluid 

 is the same as in quiet water; there is nothing to cause a disturbance. 

 "Stimulation impUes a change of relation between organism and en- 

 vironment. But if both in all their parts are moving at the same veloc- 

 ity, their relations do not change, and the conditions for stimulation are 

 wanting" (Lyon, 1904, p. 150).' The animal should then react only 

 when either it is in contact on one side with a solid, or when the current 

 is moving more rapidly on one side than on the other, producing a shear- 

 ing effect, with the necessarily accompanying disturbing action. Whether 

 this is true or not is very difficult to determine, but observation seems to 

 indicate that it is. 



'This consideration, as well as the fact that individuals resting against a surface 

 react to the current, shows the incorrectness of the theory put forward by the present 

 author (1904 h), in which stimulation was supposed to be due to the variations in pressure 

 produced through the varied movements of the animal in its spiral course. 



