1 88 The Animal Mind 



54. Orientation to Gravity: Crustacea 



That the statocyst organs in Crustacea are probably con- 

 nected with equilibrium rather than with hearing we have 

 already seen. Delage in 1887 found that Mysis, Palaemon, 

 and other forms displayed serious disturbance of equilibrium 

 when both eyes and statocysts were destroyed, showing that 

 the eyes also play a part in the maintaining of balance (180). 

 The eyes have been found to cooperate with the statocysts 

 in the fiddler crab, Gelasimus, and also in another decapod, 

 Platyonichus (127). Neither of these has statoliths. 

 Penceus membraneus, on the other hand, was found to be 

 permanently disoriented by destruction of the statocysts or 

 even removal of the statoliths, while blinding produced no 

 great disturbance, probably because of the animal's noc- 

 turnal habits (38, 250) . Young crayfish with the statocysts 

 destroyed will swim upside down as readily as right side 

 up (i u). 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 (407). 



Specific righting reactions occur in many Crustacea, 

 though in some cases these seem to be merely the incidental 

 effects of 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 



