378 



G. H. PARKER. 



any sound conclusion whatever on the question of a physiological 

 anterior. But so far as these records go, there seems to be no 

 suggestion of this state in Centrechinus. 



TABLE I. 



NUMBER OF TIMES IN TEN TRIALS ON EACH OF FOUR SEA-URCHINS (Centrechi- 

 THAT A GIVEN RAY WAS FOREMOST IN GEONEGATIVE LOCOMOTION. 



The rays are numbered according to the Loven system, the ray opposite 

 the madreporite being V, the one to the right of this ray being I. and the 

 others following in sequence around to V. 



The second conclusion to be drawn from Table I. is abundantly 

 supported by the observations. This conclusion is to the effect 

 that Centrechinus is able to carry out geonegative responses on any 

 axis in its body. This conclusion is unquestionable and shows that 

 the geotropism of Centrechinus, unlike that of a bilateral animal, 

 is performed without initial orientation and, as a type of sea-urchin 

 locomotion, it is not necessarily associated with any particular ray. 

 In this respect the geotropism of Centrechinus is much more like 

 that of a plant than like that of most animals in which the axis of 

 the animal is first moved so that the creature heads either toward 

 the center of the earth or away from it, after which locomotion in 

 the appropriate direction takes place. In Ccntrccliinus, as already 

 intimated, no initial orientation occurs, but the sea-urchin with any 

 axis forward creeps upward. This condition emphasizes what 

 Loeb long ago pointed out, the essential similarity between the 

 tropisms of plants and of animals. 



When an attempt is made to analyze the geonegative responses 

 of Centrechinus, it is found to be no simple problem. These re- 

 sponses involve stereotropism as well as geotropism and the latter 

 in its more generalized type such as occurs in plants. If a Cen- 



