spatially Determined Reactions 213 



without touching anything. Here the different parts of 

 the stream have different velocity, and pressure stimuli 

 are actually apphed to the skin. There must be pressure 

 reaction, also, when fish actually swim up-stream instead 

 of merely maintaining their places against a current (272). 

 Such a reaction was displayed, probably, by some shrimps 

 which, being in the water with the fish in the revolving 

 tank experiment, did swim against the current instead of 

 with it (448) . 



Some very interesting behavior touching on this same 

 point was observed by Garrey in a school of the little fish 

 called sticklebacks. He noted that if any object was 

 moved along the side of the aquarium containing them, 

 the whole school would move along a parallel line in the 

 opposite direction. If an individual fish happened to be 

 heading directly toward the object, it would turn in the 

 opposite direction from the one in which the object was 

 moved ; if it was heading somewhat in the opposite direc- 

 tion already, it would turn farther in that direction until 

 parallel with the object's line of motion ; if it was heading 

 somewhat in the same direction as the object, it would 

 "back off hesitatingly," and reverse itself by a turn in 

 either direction, usually taking the way around toward 

 which it was already partially headed, if the object was 

 rapidly moved, but the other way around if the object's 

 motion was slow. At first sight this behavior seems to 

 display an instinct precisely opposite to that of keeping 

 the visual field constant. Yet the sticklebacks, when 

 placed in a cylindrical glass tank inside of a black and white 

 striped vessel, moved with the latter when it moved, 

 proving that they possessed the usual tendency shown by 

 Lyon to be involved in rheotropism. Garrey points out 

 that movement in the opposite direction is produced not 



