Spatially Determined Reactions 183 



It was noted in the chapter on hearing that the peculiar 

 organs occurring in certain Ccelenterata and in many other 

 animals, which were originally called otocysts because of 

 their supposed auditory function, have had their name 

 changed to that of statocyst since it has appeared that they 

 subserve chiefly orientation to gravity. In jellyfish, re- 

 moval 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 (514, 521). It 

 has been suggested that the statocyst organs are for the 

 reception of stimuli produced by shaking, to which medusae 

 are apparently sensitive (521). Negative geotropism exists 

 in Gonionemus, which swims to the surface of the water 

 when disturbed (825). In ctenophors, 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 



(741). 



50. 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 



