Sensory reactions of Holothuria surinamensis. 251 



for when liolothurians actively eng-aged in locomotion were studied, 

 it was found that the tentacles were apparently much less sensitive 

 than when in the resting condition; in fact, even relatively strong 

 stimuli produced merely a waving about and partial retraction of 

 the particular tentacle affected, while the others were markedly 

 extended. 



Considerable variation was found in the amplitude of response 

 obtained from the tentacles of different individuals; with some even 

 the slightest stimulation leading to a maximum effect which is 

 secured in others only by repeated stimulation. After stimulation 

 with solutions of various chemicals the threshold for the mechanical 

 stimulus was considerably lowered, a maximum amplitude of response 

 then being obtained with the slightest touch. 



No differences could be detected in the reactions of the different 

 tentacles, nor in different regions of the same tentacle, though 

 it might have been expected that the outer surface of the discs 

 would have been more sensitive than the shafts. 



4 



Fig. A. Stages in the contraction of a papilla (2 : 1). 



1. A fully extended papilla. 2. First stage in contraction — response to a sLight 



touch, at X. 3. Second stage in contraction — response to more intense stimuli. 



4. Complete retraction of papilla repeatedly stimulated. 



b) The podia. As nearly as could be determined, the pedicels 

 and papillae were of equal sensitivity to tactile stimuli. In both 

 cases the reaction time of podia in the mid-body region was slightly 

 greater than that of the tentacles, being 0,4 — 0,6 seconds at 

 240—27°; for stimuli of approximately equal intensity the amplitude 

 of the response was much less than that of the tentacles in non- 

 moving animals. 



The papillae are more scattered than the pedicels and could 

 readily be studied singly under a binocular microscope. They 

 showed several stages of reaction to tactile (and other) stimuli. In 

 general, the mode of retraction was by contraction; but the extent 

 of the retraction varied with the intensity of the stimulus, and the 

 following well-deflned steps were made out (Fig. A): (1) shows the 



