THE RELATION BETWEEN LIGHT AND PIGMENT-FORMATION. 561 



window, with several coats of pure paiut. Spectroscopic 

 tests showed that the red was pure and that the green paint 

 reflected only a trace of blue in addition to the whole of the 

 green light. These vessels were kept under a water-circula- 

 tion and faced a south light. Finely divided pieces of 

 Ceramiuni were employed as food. The Hippolyte used 

 wei*e small, almost colourless specimens, similar to those 

 employed in the other experiments on coloured light. 



The results of exposure to these monochromatic back- 

 grounds was very decisive (Table IV, p. 579). Upon the green 

 one the development of pigment was arrested. The Hippo- 

 lyte assumed a semi-nocturnal (green) tint, and remained with 

 the red pigment contracted throughout the experiment. This 

 green colour is, however, not retained if the background is 

 changed. Under these circumstances the animals revert to 

 the pale red-lined colour variety which they exhibited 

 initially. Upon the red background, on the other hand, the 

 red and yellow pigments had considerably developed, and after 

 a month's exposure gave a bright orange-red tint to the 

 specimens, and tiiis persisted after change to other back- 

 grounds. It would be of interest to know whether Minckie- 

 wicz (1907-8), who has also obtained results of this kind 

 with Hippolyte, tested the permanent or transient nature 

 of the induced colouring. 



IV. The Food of Hippolyte as a Possible Source of 



Pigment. 



The relation of Hippolyte varians to the alga3 of its 

 choice is a distinctive one. The peculiar features of this 

 species, the range and cryptic character of its variable colora- 

 tion, its choice of, and tenacity of hold upon its weed, its 

 distribution, and its food are all bound up with the presence 

 of these plants. It is possible that Idothea and some 

 Amphipods ai-e equally intimately related to their habitat, 

 but among macrurons Decapods Hippolyte varians is 

 probably unique in this dependence upon its algal environ- 

 ment. 



