Sensory reactions of Holotliuria surinamensis. 261 



the usual withdrawing reactions when the drops were made to fall 

 over them. 



Holoihuria with the anterior parts amputated gave the same 

 series of reactions to vibrations, but were more easily exhausted, 

 not more than 5 successive reactions being obtained to shocks coming 

 at one minute intervals. 



IV. Light. 



1. Introduction. 



The radial arrangement of the body in starfishes and sea urchins 

 has given a special interest to the study of their reactions to light 

 in connection with theories of phototaxis which have been advanced 

 on the basis of work with bilaterally symmetrical animals. In echino- 

 derms other than holothuriaus there is no good evidence of orientation 

 produced by light. With regard to the holothuriaus Mast (1911, 

 p. 211) says: — "The lack of orientation in moving from a source 

 of light is much more striking in the holothuriaus, which are super- 

 ficially at least much more definitely bilaterally symmetrical" than 

 the starfishes ; this generalization is based on Peaese's (1908) account 

 of the behavior of TJiyone. So far as known the holothuriaus are uni- 

 formly negative in their reactions to light, and since the time of 

 Daltell (1851, Vol. I) have been spoken of. as nocturnal animals; 

 Stichopus is an apparent exception, for it is seen in great numbers 

 on brightly illuminated bottoms, but even here some species at 

 least crawl into dark places after the passage of the breeding season 



(MlTSUKUEI, 1903). 



2. Phototropism. 



Holothuria surinamensis, captiva and ratlibuni all move away from 

 a source of light, in a manner strikingly at variance with that de- 

 scribed for Tliijone. As has been brought out in the discussion of 

 locomotion (page 242), Holothuria was not observed to ci^awl in any 

 other way than with th'e anterior end to the front, while Tkijone 

 moves away from the light, like a sea urchin (Holmes, 1912), with 

 any angle of the body in advance. When a number of holothuriaus 

 were placed in a vessel near a window, though not exposed to the 

 direct rays of the sun, they oriented away from the light and moved 

 to the side of the vessel farthest from the window. Having gotten 



