206 PERCEPTION OF LIGHT 



object, we can hardly doubt that the absence of light is 

 the principal guide. 



Professor Graber,* in his interesting memoir on 

 this subject confirms the observations on ants and 

 Daphnias, in which I showed that they are sensitive to 

 the ultra-violet rays, by similar observations on earth- 

 worms, newts, etc. It is interesting, moreover, that the 

 species examined by him showed themselves, like the 

 ants, specially sensitive to the blue, violet, and ultra- 

 violet rays. Graber, however, states^that he differs 

 from me inasmuch as I attribute the sensitiveness to 

 the ultra-violet rays exclusively to vision ; — that it is 

 " ausschliesslich durch die Augen vermittelt." I am 

 not, however, of that opinion as a general expression, 

 though I believe it to be true of ants, where the 

 opacity of the chitine renders it unlikely that the light 

 could be perceived except by the medium of the eyes 

 or ocelli. 



Graber has shown in earthworms and newts, and 

 Plateau t in certain Myriapods, that these animals 

 perceive the difference between light and darkness by 

 the general surface of the skin. But more than this, 

 Graber seems to have demonstrated that earthworms 

 and newts distinguish not only between light of differ- 

 ent intensity, but also between rays of different wave- 

 lengths, preferring red to blue or green, and green to 

 blue. He found, moreover, as I did, that they are 

 sensitive to the ultra-violet rays. Earthworms, of 

 course, have no eyes; but, thinking that the light might 



* "Fundamental Versuche iiber die Helligkeits und Farben Em- 

 pfindlichkeit augenloser und geblendeter Tbiere," Sitz. Kais. Ahad. 

 d. Whs. Wien: 1883. 



t Journ. de VAnatomie et de la Physiologie, 1886. 



