Sensory reactions of Holothuria surinamensis. 265 



3. The shadow reflex. 



Ill the course of several hundred trials, no reflexes other than 

 the slow system of movements producing negative phototropism were 

 observed when the intensity of the light falling on Holothuria was 

 suddenly increased. This was true for individuals which had 

 been in darkness for some hours, as well as for those which had 

 been in light of low intensity for even longer periods. The partial 

 retraction of the tentacles when illuminated by a spotlight was much 

 slower than the shadow reflex presently to be discussed, and was not 

 of a definitely predictable character; it appeared for the most part 

 as a general waving about of the tentacle stimulated. 



All the holothurians which I have examined give, however, a 

 well defined response to sudden diminution of light intensity, a 

 reflex which forms a feature of the behavior of other holothurians 

 also, e. g. Cucumaria (Grave, 1905) and Thyone (Pearse, 1908). 

 Shading the whole animal produced retraction of the tentacles, 

 closure of brim and cloacal sphincter, and a general contraction, 

 especially of the anterior and posterior ends. Following this, the 

 parts thus contracted would soon expand, and remain expanded in 

 the shade. The shadow reaction was most easily studied in the 

 case of the tentacles, where the reflex consisted of the withdrawal 

 of the tentacles, the closure of the brim, and a bending of the body 

 to the side away from that shaded. The extent to which the 

 tentacles retracted, the time during which they remained concealed 

 if complete withdrawal had been secured, and the degree to which 

 the body muscles were implicated in the reaction, were in a measure 

 determined by the area of the region shaded, the sharpness of the 

 shadow edge (i. e., the rate of intensity reduction), and the actual 

 amount of light reduction accomplished by the shading. Single 

 tentacles were caused to disappear within the partially constricted 

 collar by shading them individually, either at the peltate tip or at 

 any point along the shaft. 1 did not determine the minimum shadow 

 area which would produce this reaction. 



The whole surface of Holothuria is sensitive to sudden shading. 

 With the tentacles completely retracted and the brim closed down, 

 well defined reactions were obtained from the anterior end, especially 

 from the brim. The rim of the cloaca was particularly sensitive; 

 very slight reactions were had from the posterior end when this 

 rim was concealed by the closure of the cloaca, or when it was 



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