THE RELATION BETWEEN LIGHT AND PIGMENT-FORMATION. 567 



precise, and it would certainly lielp to account for the 

 crimson and yellow colouring found in deep-waterand shallow- 

 water Hippolyte respectively. 



The results, then, of these two factors, the action of diffused 

 coloured light and that of backgrounds in white or mono- 

 chromatic light, are not contradictory. They are the two 

 factors which, so far as we yet know, are associated in the 

 production of pigmentation in Hippolyte. The green speci- 

 mens on Zostera are green, not only because they are on a green 

 background in bright or fairly bright light, but because at or 

 near the surface of the sea the red rays are most potent, and 

 their action is to produce that network of yellow pigment in 

 Hippolyte, which is the basis not only of green tints but of 

 those yellowish tints that this animal assumes on the etiolated 

 parts of Zostera, and of the bi-own specimens on various 

 brown weeds so characteristic of the Plymouth littoral flora. 

 The diffuse red light, on penetrating to more densely absorb- 

 ing backgrounds, such as coarser brown weeds, is checked in 

 its action upon Hippolyte by the tendency for such back- 

 grounds to produce red pigment in them. Hence the absence 

 in s^^ch cases of that more precise colour-relation to the inci- 

 dent light. The brown pigmentation contains many red and 

 yellow chromatophores, but the red is scarlet and not the 

 crimson of the deeper zones. 



Passing out of the range of the action of the red rays, the 

 characteristic zone of the Florideae is encountered, and it is 

 in this zone that the green rays are more potent. Their effect 

 in producing crimson pigmentation is seen in parti-coloured 

 specimens of the red-lined variety and in occasional pink 

 specimens of the Laminarian zone, but it is not until a fair 

 depth is encountered that their action is made clear by the 

 dominance of this peculiar carmine pigment, which has 

 hitherto been confused with the vermilion or scarlet one 

 under the confusing term " red." No doubt there are similar 

 effects of yellow, orange, and blue rays to be analysed before 

 a full analysis of the coloration of Hippolyte can be given. 

 The main conclusion derived from these experiments is that 



