THE GEOTROPISM OF THE SEA-URCHIN CENTRECHINUS. 377 



be shown that the ray in the sand-dollar that is opposite the anus 

 is the homologue of the ray in Astcrias that is to the left of the 

 madreporite and that serves as the director in locomotion. Hence 

 the axis of symmetry and direction of locomotion in Mellita, as 

 determined by Crozier, are in agreement with the conditions in 

 Asterias as determined by Cole. This is a rather striking corre- 

 spondence in both anatomical and physiological details. 



Although the radial symmetry of the echinoderms expresses 

 itself in the ability of these animals to move in any direction in 

 the horizontal plane, yet in the few instances studied there seems 

 to be also a marked tendency toward a physiological anterior which 

 takes its origin from the position of the madreporite and is in 

 agreement with the structural bilaterality of the spatangoids and 

 the clypeastroids. 



To test these relations in the regular sea-urchin Centrechinus a 

 bit of white thread was tied to a spine on the ray to the left of the 

 madreporite (III. in the Loven system) and, with this ray thus 

 identified, the sea-urchin was allowed to climb the sides of a glass 

 jar ten times and the position of the axis of locomotion in relation 

 to the given ray was recorded for each ascent. Tests of this kind 

 were made on four animals. These animals, after an ascent had 

 begun, showed little of the circling movement observed by Crozier 

 (1920^) in Mellita, but maintained a fairly constant relation be- 

 tween the axis of locomotion and their structural axis over a verti- 

 cal course of some 40 centimeters. It was not always easy to 

 observe the exact relation of the structural axis of the animal, as 

 indicated by the marked spine, to the direction of locomotion, but 

 this relation is accurately enough indicated in terms of rays, though 

 the animals may have crept at times more nearly interradially than 

 radially. Records were kept by noting which ray was nearest the 

 physiological anterior during the test. The results are shown in 

 Table I. 



Two conclusions may be drawn from Table I. First, there is 

 nothing about the records in this table that suggests that Cen- 

 trechinus has a physiological anterior. The ray III., that might 

 be suspected in this respect, is in no sense a director, in fact it is 

 rather the reverse. It is, however, perhaps open to doubt whether 

 there is a sufficient number of observations in the table to warrant 



