158 The Animal Mind 



subserve chiefly orientation to gravity. In jellyfish, removal 

 of these organs does not seem to affect the animal's power of 

 keeping its balance; apparently equilibrium is maintained 

 here by the simple action of gravity, for dead jellyfish float 

 in the right-side-up position (286, 291). It has been sug- 

 gested that the statocyst organs are for the reception of 

 stimuli produced by shaking, to which medusae are ap- 

 parently sensitive (291). Negative geotropism exists in 

 Gqnionemus, which swims to the_ surface of the water when 

 disturbed (470). fnTctenophors, the statocyst organ, which 

 is usually at one pole of the body, has been found to function 

 as an organ for the maintenance of equilibrium (415). 



58. Orientation to Gravity: Planarians 

 A good example of a specially developed reaction having 

 for its result the " righting" of an animal in an abnormal 

 position is offered by the behavior of a planarian that has 

 been turned over so that its back rests on the surface of 

 support. The reaction consists of a turning of the body, 

 beginning with the head end, about the long axis, so that 

 a spiral form is assumed. The dorsal surface of the animal 

 is convex, the greatest thickness of the body being in the 

 middle line. When the planarian lies on its back, it thus 

 naturally tips to one side, like a keeled boat out of water. 

 This side, being brought into contact with a solid, gives 

 a reaction analogous to the negative one, that is, it extends 

 or stretches. Such a stretching of one side when the planarian 

 is right side up would of course produce a turning in the 

 opposite direction, a negative reaction. In this case, however, 

 the opposite side does not contract to allow of turning, but 

 maintains the same length. The necessary result is that 

 the body is thrown into a spiral: as soon as the ventral 

 surface of the head comes into contact with the solid, in 



