THE GEOTROP1SM OF THE SEA-URCHIN CENTRECHINUS. 379 



tr echinus is dropped so as to rest with its side or its aboral face 

 on the horizontal floor of an aquarium, an activity of spines and 

 ambulacral feet ensues which eventually rights the animal in that 

 its oral side is brought next the aquarium floor. Not till righting 

 has been accomplished does locomotion in the proper sense begin. 

 I never have seen a Centrechinus progress on its back or its side, 

 but only with its oral surface next the substrate. It has often 

 been assumed that in echinoderms the righting reaction is a re- 

 sponse to gravity, but this is doubtful, in Centrechinus at least, 

 as the following experiment shows. If a strong thread is tied 

 rou-nd the equator of a Centrechinus and the animal is suspended 

 in an aquarium so that its side touches the vertical glass wall, the 

 spines and feet will begin the same kind of movements that they 

 did when the animal rested on its side at the bottom, and in a short 

 time the sea-urchin will have its oral surface applied to the vertical 

 face of the aquarium as it formerly did to the floor. Since in this 

 test the axis of the sea-urchin is at right angles to the direction of 

 gravity instead of being parallel to it as in the ordinary righting 

 reaction, it is clear that righting is not a response to gravity, but 

 to a solid surface against which the creature comes always to 

 apply its oral side. Righting in Centrechinus, therefore, though 

 apparently geotropic, is in reality not so, but stereotropic. On 

 this point my results confirm those of Moore (1910), who has 

 described a similar condition in the starfish. In discussing the 

 geotropism of Centrechinus, therefore, the righting movements are 

 not to be considered, for they belong to a different category of 

 reactions ; they are reactions to solid surfaces. In this respect 

 Centrechinus agrees with Planaria, whose righting reactions are 

 apparently also stereotropic (Pearl, 1902), though this worm like- 

 wise shows true geotropism (Olmsted, 1917). 



After a Centrechinus has obtained a footing on a vertical sur- 

 face, it does not wander indiscriminately over this surface, but it 

 travels up the surface against gravity. It is this reaction that is 

 indicative of true geotropism and that still requires to be consid- 

 ered. 



Two general theories have been advanced to show how geotropic 

 responses are accomplished : the so-called mechanical theory appar- 

 ently first proposed by Aderhold (1888) and advocated by Ver- 



