23O A. T. CAMERON AND C. H. o'DONOGHUE. 



a greater range of dilutions of the chemical compounds employed 

 would yield more definite results; e.g., our results for certain 

 species of shrimps are inconclusive, and we have been unable to 

 demonstrate " darkness " responses with narcotized sea-urchins, 

 though these have been reported for the normal animal. 



In addition to the normal " shadow " responses quoted in the 

 introduction may be mentioned here the well-known reaction of 

 frogs to shadow (immediate diving) and that of cryptobranchiate 

 nudibranchs (retraction of the gill-plumes within the branchial 

 collar). Similarly Englcna viridis sinks. 



The response itself is obviously adaptive (i.e., of survival value, 

 and so likely to have been accentuated by selection), since under 

 natural conditions a shadow suddenly falling on an animal implies 

 the interposition of a solid object between it and the light i.e., a 

 probable enemy. It is extremely well developed in the pelagic 

 fishes, as we found to our cost when trying to catch them. This 

 normal response becomes less marked in all animals when exam- 

 ined in the laboratory under experimental conditions. 



The peculiarity of the response in the narcotized animal seems 

 to consist in its accentuation, actually, or relatively through t!ie 

 abolition of volitional movements and other reflexes, so that its 

 manifestation can be controlled experiment ally. 



The reaction is always definitely a reflex, though the animal may 

 be by no means in a condition of complete narcosis. While cessa- 

 tion of a very intense illumination undoubtedly produces greater 

 effects than cessation of a lesser degree of it, the sudden change is 

 the controlling factor, and is quite effective when made from a 

 brilliant to a dull but distinct light. In almost all cases it is neces- 

 sary to allow the animal to rest in dull light for a minute or two 

 between tests, as after repeated subjection to strong illumination 

 the response ceases. This is obviously connected with fatigue 

 due to photochemical changes of the retinal pigment present alike 

 in vertebrates and arthropods. While undoubtedly the retina is 

 the receptor of the stimulus in animals possessing it, the normal 

 reactions of sea-urchins suggest that their color spots can function 

 in a similar way. 



Increase of illumination either produces no effect or one much 

 smaller than a similar decrease. There were one or two excep- 



