Sensory reactions of Holothuria surinamensis. 257 



sideward bending-s, etc., and no trace of aboral nervous ring has 

 "been identified among the liolothurians. 



2. Continued tactile stimulation 



as a factor in the behavior of Holothuria was sought in evidence 

 for stereotropic and similar forms of irritability. 



a) Stereotropism; and the righting reaction. Holo- 

 thurians were sometimes found to have lodged themselves in narrow 

 holes and crannies in rocks, and this, taken in connection with the 

 burrowing habit, seemed to point to a general stereotropic sensiti- 

 vity of the animal's surface. In aquaria however, they showed no 

 tendency to collect in groups, not even in the angles between walls 

 or in the corners. When fitted into narrow glass tubes so that 

 the anterior end projected through one opening, they did not tend 

 to maintain this position, but soon crawled out of the tubes. Simi- 

 larly, experiments designed to discover the existence of rheotropism 

 in Holothuria gave negative results. The animals moved either 

 with or against the current flowing down a narrow trough, de- 

 pending on circumstances not connected with the current itself, 

 save when the water current was violent, as then the effect was 

 comparable to strong mechanical stimulation, producing sideward 

 turning away of the anterior end and gradual movement until out- 

 :side the zone of disturbance, but without any orientation. 



Echinoderms generally, are capable of righting themselves when 

 turned over on the dorsal surface. It is known that the righting 

 reaction of the starfish is due to the stereotropism of its tube feet, 

 rather than to any geotropic control of the dorso-ventral orientation 

 ■of the body (Mooee, 1910). Synaptids possess definite "positional 

 organs", which function as statocysts, but no such organs occur 

 among other liolothurians, and the following experiments show that 

 Holothuria in its righting behavior is comparable to the starfish — 

 or better, to an isolated starfish arm. 



The righting reaction of H. surinamensis involves two distinct 

 factors — 1. the dorso-ventral curvature of the body, and 2. the 

 twisting of the animal on its long axis, forward crawling, and the 

 gradual attachment of its pedicels to the substratum. The previ- 

 ously described tendency to throw the body into a curve when 

 turned over on its back or otherwise removed from its normal 

 position makes it impossible for Holothuria to remain on its dorsal 

 surface, but causes it when so placed to topple over to one side or 



