Sensory reactions of Holothuria surinamensis. 273 



This is shown by the facts, (1) that the rate of light reduction is 

 a factor in the stimulus, for to be successful in producing the reaction 

 shadows had to have a sharp edge, (2) that with two sources of 

 light — e. g., a flash light and a lamp — extinguishing one of 

 them was under certain circumstances a stimulus for the shadow reac- 

 tion (1) and (3), that the amplitude of the reaction varied, directly 

 with the intensity of the light from which the animals were shaded. 

 No means was at hand for expressing the last relation quantitatively. 

 Another aspect of the shadow reaction is the converse of the 

 familiar colored-light problem in phototropism : is the shadow re- 

 action initiated by the cutting out of any particular set of wave 

 lengths? No conclusive experiments were made with Holothuria, but 

 with Balanus it was found that cutting out either the blue or green 

 was sufficient to produce the typical shadow reflex. The colored 

 lights were secured by the use of glass ray filters, so the intensity 

 relations were not clearly evident; but to the human eye the blue 

 glass was less transparent than the red. When the barnacles had 

 reexpanded under the colored lights, cutting out the red or yellow 

 produced no eifect, while [shading through the blue and green 

 glasses gave typical reactions.^) The cutting out of the ultra-violet 

 is certainly not a factor, as the shadow reaction was given under 

 glass, and the sudden interposition of a glass plate between the 

 holothurians and the sun produced no effect. 



4. Discussion. 



Holothuria has been shown to be photokinetic, negatively photo- 

 tropic when there is a horizontal component of the incident light, 

 and to be reactive over its whole surface to rapid decrease in light 

 intensity. No special organs of light reception are known among 

 the holothurians, excepting the so-called light detectors at the base 

 of each tentacle in Synapta hydriformis (Claek, 1907) and a few 

 other species. Photic sensitivity in Holothuria, as in many other 

 echinoderms, is a general integumentary function (Mangold, 1909, 



1) Appropriately controlled, this method might well be used to 

 determine the liminal stimulus at different temperatures, etc. 



2) Ewald (1912) found that the larvae of Balanus jierforahcs were 

 affected by sudden changes in the intensity of light, and that the green 

 and yellow-green were the most potent in evoking these reactions as 

 well as in effecting orientation. 



