58 PHOTORECEPTION IN KOLOTHURIA 



The time required for the orientation of individuals 6.0 to 6.5 

 cm. long by sunlight reflected from a mirror, was found to be 6 to 8 

 minutes at 20°C.; the animals were at the beginning placed at right 

 angles to the incident light. Other things constant, the speed of 

 orientation varies inversely with the light intensity. The same in- 

 dividuals, and others in addition, were then in a similar manner 

 exposed to sunhght which had passed through an alcoholic solution of 

 the integumentary pigment of this species. A thickness of solution 

 was used sufhcient to produce apparently complete absorption of 

 wave-lengths shorter than SSOix/jl; the pigment of Holothuria captiva 

 does not exhibit a band spectrum {cf. Crozier, 1915). 



It was found that under the influence of the filtered sunlight, orien- 

 tation required on the average 6 minutes — the same time as noted 

 with the unfiltered light. 



The photodynamic activity of eosin exhibits a pecuHarity, dis- 

 cussed by Huber (1905) and by Clark (1918-19), which corresponds 

 in certain respects with the apparent failure of a screen of Holothuria 

 pigment to "protect" the animal to any degree from the stimulating 

 action of Hght. The photodynamic action of eosin on rennin is found 

 not to be materially lessened by the interposition of an eosin screen; 

 hence the fluorescence of the photodynamic agent cannot be regarded 

 as essentially concerned in any way with the production of the toxic 

 effects. Clark (1918-19) has suggested that in such cases the essen- 

 tial photodynamic action (production of toxic materials) may depend 

 upon wave-lengths for which the fluorescent substance is partially 

 transparent. 



Light which has passed through an adequate layer of Holothuria 

 pigment will not excite fluorescence in a subsequent layer, hence the 

 fluorescence must be due to wave-lengths readily absorbed. But 

 light filtered in this way is not detectably weakened as a stimulating 

 agent^ for this animal. It must therefore be presumed that, if the 

 pigment is in reality implicated in photoreception, light not readily 

 absorbed by it is concerned in certain photochemical transforma- 

 tions differing from those which find expression in dichromaticity. 



1 Note that in this experiment the criterion of stimulating power has to do 

 with the rate of orientation of the tested organisms — not with the rate of loco- 

 motion. 



