spatially Determined Reactions 211 



§ 62. Orientation to Other Forces 



One force, which, as was noted in Chapter III, produces 

 orientation, namely, the electric current, we shall leave out 

 of accoimt. It is not a stimulus to which animals are nor- 

 mally subject, and though its action on living matter is of 

 great interest to the physiologist, the comparative psycholo- 

 gist's difficulty in finding a psychic interpretation for the 

 facts may Justify setting them aside. Similar considera- 

 tions apply to orientation to centrifugal force. There 

 remain the orientations that have been termed respectively 

 "rheotropism" and "anemotropism," responses to cur- 

 rents of water and to currents of air. 



The tendency shown by many aquatic animals to orient 

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

 current, was formerly thought to be a response to the pres- 

 sure exerted by the current — a reaction leading the ani- 

 mal to resist pressure. Lyon, however, pointed out that 

 this explanation 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 sur- 

 rounding the animal flowed with uniform velocity in all 

 its parts, no stimulus whatever could be exerted by the 

 water pressure (448). It seems probable that eyeless 

 animals do not, as a matter of fact, orient themselves against 

 a current of this sort, and that rheotropism in their case 

 occurs when a current of unequal velocity disarranges 

 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 inter- 

 fere with the normal backward stroke of the cilia, causing 



