380 G. H. PARKER. 



worn (1889) and the statocyst theory put forth by Lyon (1905) 

 and espoused by a number of later workers, Kanda (1914) and 

 others. According to the mechanical theory the weight of the 

 body of the responding organism is so disposed in relation to the 

 locomotor apparatus that the creature is kept mechanically headed 

 either toward the center of the earth or away from it. In this 

 way the action of gravity on the body as a whole determines the 

 direction of locomotion. In such cases the organism would main- 

 tain the same orientation dead or alive. Few, if any, instances of 

 this kind have ever been found, and though it must be admitted 

 that every organism is always under the direct influence' of gravity 

 and varies in weight in different parts of its body, practically none 

 have been found that exhibit conditions favorable to this view. 

 Centrcchlnus with its symmetrical distribution of parts gives no 

 suggestion of a preponderance of weight on one side or the other, 

 and the fact that it may assume a new axis with almost every 

 ascent on the aquarium wall is a condition extremely difficult for 

 the mechanical theory to meet. 



The statocyst theory assumes that within the body of the organ- 

 ism there are movable masses of higher specific gravity than their 

 fluid surroundings, and that these masses under the influence of 

 gravity press upon one side or the other of their containing cham- 

 bers in accordance with the position of the organism in relation to 

 the center of the earth. To these pressures the organism then 

 responds in an appropriate way. Organs acting in this fashion 

 and known as statocysts with their contained statoliths occur com- 

 monly among such animals as the worms, the mollusks, and the 

 crustaceans. 



This theory is also believed to apply to the simpler animals and 

 to the plants in that geotropism is excited in these organisms by 

 the pressure exerted by small particles contained within the vacu- 

 oles of their protoplasm. This view is supported by such observa- 

 tions as those of Zollikofer (1918) on the seedlings of certain 

 composir.se. When these seedlings are placed in the dark for three 

 or four days, the starch grains in their hypocotyls disappear, and 

 with the disappearance of the starch the geotropic reactions of the 

 seedlings cease, though the seedlings seem not to have suffered 

 from the treatment, for they are still phototropic. Apparently the 



