Sensory reactions of Holothuria surinamensis. 277 



If we agree that any change in the concentration of the photo- 

 sensitive material may supply a stimulus for a negative reaction, 

 in the manner above described, it is readily understood why the 

 reaction to shading is more vigorous and more promptly carried out 

 than is the reaction to an equivalent increase in light intensity. 

 On this basis we may also account for the following facts: 



1. Photic sensitivity is distinct from that to mechanical and 

 chemical stimuli. 



2. The sensitivity to light and to shading are together dis- 

 tributed over the whole surface of Holothuria. 



3. The shadow reflex is more pronounced in bright light. 



4. The shadow reflex disappears above a certain temperature viz.,. 

 the temperature above which the reconstructing reactions, already 

 accelerated by the higher temperature, are no longer perceptibly 

 increased in rate by shading, (The purely photolytic efl'ects have 

 no positive temperature coefficient.) 



Increased reactiveness to shadows after the first few stimula- 

 tions may be explained as a consequence of the photochemical 

 equilibrium needing several "shocks'' to get it started beyond some 

 critical value, a sort of photochemical induction. Other features of 

 the photic reactions of Holothuria may be accounted for in a similar 

 manner. 



There is some evidence that the fluorescent pigment is 

 itself the photo-sensitive material: it absorbs the green and blue 

 wave lengths, the part of the spectrum which is chemically the 

 most active and which is generally the most powerful in producing 

 phototropic orientation among animals (cf. Loeb, 1905; Gross, 1913) 

 and in causing the shading reflex; it was found to be decomposed, 

 slowly it is true, by light and air acting on alcohol solutions ; tests 

 of the integument of live Holothuria showed an abundant catalase 

 reaction; so that it is not at all impossible that the pigment is 

 oxidized under the influence of light in nature. The anatomical 

 conditions are entirely favorable to this view; Polaea (1906) has 

 described and figured the situation of the pigment cells in the in- 

 tegument of H. poli, and indeed has suggested that they function 

 as organs of light reception. These anatomical relations are as 

 follows: in the epidermis are found scattered sense-cells, which are 

 connected with ramifications of integumentary nerves-strands arising 

 from the ambulacral nerves; the mesenchj^mal cells of the super- 

 ficial connective tissue are crowded with pigment, and are con- 



