1 62 The Animal Mind 



But the prettiest evidence for the static function of the 

 statocysts was obtained when powdered iron was substituted 

 for the mineral bodies in the open statocysts of Palaemon. 

 It was found that when a magnet was brought near, the animal 

 would respond by taking up a position corresponding to the 

 resultant of the pull of the magnet and that of gravity (226). 



Specific righting reactions occur in many Crustacea, 

 though in some cases these seem to be merely the incidental 

 effects of their ordinary locomotion. Branchipus, the fairy 

 shrimp, normally swims upside down; if turned right side 

 up when moving along the bottom of the vessel, it continues 

 to move in this position without showing any disturbance 

 until it happens to rise a little from the bottom, when ap-^ 

 parently the weight of the body pulls it around into the 

 usual upside-down position. The crayfish has two methods 

 of righting itself: a quick "flop" executed with the tail, 

 and a slow and laborious raising of itself on one side and 

 tipping over (96). 



Many Crustacea show marked responses to gravity: 

 for example, Parker found decided negative geotropism in 

 the females of the marine copepods whose depth migrations 

 he studied. It seems to be needed to counteract the tendency 

 of the animals to fall to the bottom by their own weight (304). 

 In certain copepods, light was observed to change the sense 

 of the response to gravity, not by taking its place as a directive 

 stimulus, but apparently by producing some physiological 

 change in the animals. Their normal geotropism was 

 positive, that is, they had a tendency to move downwards. 

 In darkness, however, their geotropism became negative. 

 They were also negatively phototropic to strong light. If, 

 when in the negatively geotropic phase, they were illuminated 

 from below by intense light, from which they would ordinarily 

 have moved away, the change from negative to positive 



