REACTIONS OF MELANOPHORES 203 



Seeing larvae when reared over a white bottom are very pale 

 in appearance, due not only to the contracted condition of the 

 melanophores, but also to the fact that the number of the melan- 

 ophores is not so great. Eyeless larvae reared over no matter 

 what bottom are darker than these, the melanophores being 

 always expanded in the light. But, although the number of the 

 melanophores is greater than the number in seeing larvae on 

 white backgrounds, it is smaller than the number in seeing 

 larvae under the general environmental conditions of the aver- 

 age aquarium dark bottom described above, and over a black 

 one. 



That the reactions of the melanophores are adaptive is shown 

 by the above observations and also by the following. Seeing 

 and eyeless larvae were placed when young (25-30 mm. long), 

 in a large aquarium containing Elodea and other water plants. 

 Coarse, brownish sand composed the bottom. As time went 

 on, with the sinking to the bottom of decayed leaves and stems 

 and the collected faeces of the animals, the bottom became 

 a very dark brown, almost black, over which the seeing larvae 

 were relatively inconspicuous, while the eyeless larvae 

 were distinctly to be seen. Periodically, the aquarium was di- 

 vided into two parts by a plate of glass placed diagonally across 

 it and the nature of the bottom in one half changed by sprink- 

 ling white sand over it. The seeing larvae promptly changed 

 over the 'white' bottom, becoming markedly paler, due to the 

 contraction of the melanophores. The melanophores of the eye- 

 less larvae did not respond. When the dividing plate of glass 

 was removed the difference between the seeing larvae that have 

 been over the 'white' bottom and those that have been over the 

 dark one became very apparent, while the eyeless were alike 

 over both bottoms, and dilTerent from each of the kinds of see- 

 ing larvae, being darker than the seeing larvae that had been 

 over the 'white' bottom, and paler than those that had been 

 over the dark bottom. 



No further evidence regarding the cause of the secondary re- 

 sponses of the melanophores of seeing larvae to light and dark- 

 ness is advanced beyond that which has been earlier expressed. 



