Spatially Determined Reactions 161 



harder time of it, and does not succeed in pulling itself over 

 unless it is perfectly fresh and vigorous. It occasionally 

 rests for some time when it has reached a position of stability 

 halfway over, before continuing the process (365). 



Lyon has observed marked negative geotropism in the 

 larvae of the sea urchin. He was unable to test Davenport's 

 theory of the nature of the geotropic response by putting the 

 animals in a solution of the same density as their own bodies, 

 for the reason that such a fluid was too dense and sticky 

 (being made of gum arabic and sea water) for them to swim 

 in. That the response was merely a passive one he 

 thinks improbable, because the larvae from eggs that have 

 been rapidly rotated, or "centrifuged," as it is called, have 

 all the pigment on one side of their bodies and may therefore 

 be supposed to have their ordinary balance disturbed; yet 

 they rise to the surface just like the rest (256). 



62. 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 e^es_also play a part in the maintaining of balance (97). 

 The eyes have been found to cooperate with the statocysts 

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

 Platyonichus (78). Neither of these has statoliths. Penaus 

 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 dis- 

 turbance, probably because of the animal's nocturnal habits 

 (19, 138). Young crayfish with the statocysts destroyed 

 will swirn upside down as readily as right side up (71). 



