THE GEOTROPISM OF THE SEA-URCHIN CENTRECHINUS. 381 



starch grains act as statoliths in the spaces in which they are 

 lodged. In this way the small solid particles in the protoplasm of 

 the simpler animals and plants are supposed to render these organ- 

 isms geotropic. 



As a refinement and extension of this view, Small (1920) has 

 suggested that under gravity the disperse phase in the protoplasm 

 of geotropic organisms moves to one side of their bodies, a process 

 analogous to creaming, and thereby brings about a change in elec- 

 trical potential which results in geotropic movements. As an ob- 

 jection to this view Blackmail (1921) points out, among other 

 things, that creaming is a slow process, and that geotropic response 

 is often quick. Certainly in Centr echinus such a response can 

 occur in less than a second, as can be shown by tilting into the 

 vertical a horizontal plate of glass on which a sea-urchin is creep- 

 ing. Hence creaming can scarcely explain the geotropism of the 

 sea-urchin. 



How this is to be explained is not a simple matter, for Cen- 

 tr echinus, so far as is known, is entirely devoid of statocysts, and 

 even assuming that it possessed them, the fact that it progresses 

 upward with any axis forward is a difficult feature to explain. 

 Nevertheless its body is provided with a number of parts whose 

 action may make clear how its geotropism is accomplished. Such 

 parts are the spines and the ambulacral feet. They are heavier 

 than the sea water in which they are immersed and in consequence 

 of their weight tend to hang down from their supports. This is 

 especially true of the spines, which are heavily impregnated with 

 lime and provided with ball-and-socket bases. In fact, these or- 

 gans are beautifully arranged to respond to the pull of gravity and, 

 assuming that their bases are provided with a nervous mechanism 

 sufficiently differentiated for the purpose, they might perfectly well 

 serve as organs for the initiation of the response to gravity. In 

 that case the stimulus would be the deforming pressure exerted at 

 the base of the spine by its movement under the pull of gravity. 

 It is a deforming pressure of this kind that acts as a stimulus and 

 not a general pressure such as fluids exert and as was believed to 

 be effective in geotropism by Jensen (1893). A deforming pres- 

 sure, if exerted locally at the base of the spine, might well excite 

 in a differentiated system of basal receptors impulses to locomotion 



