562 F. w. (JAMJMj;. 



In former papers on the subject, the relation existing 

 between the pigments of Hippolyte and tlie coloration of its 

 surroundings was explained as due to light effects, as if the 

 weed backgrounds in virtue of their disposition, of their 

 luminous character, and colour, acted as stimuli to tlie chroma- 

 tophores of the prawn. However, before we accept that 

 explanation, the influence of two other factors must be con- 

 sidered : First, the effect of darkness on pigment-formation, 

 and second, the source of these pigments, whether derivative 

 or not. The first factor — darkness — is discussed on pp. 577- 

 579, and it is there shown that the red (vermilion) pigment 

 does not i-equire the stimulus of light for its development, and 

 that it increases in amount when the Hippolyte are kept in 

 darkness. The yellow pigment, however, is more dependent 

 on light for its formation and increase, diminishing in amount 

 in specimens kept in darkness, especinlly if little or no food is 

 supplied to them. The crimson pigment and the diffuse blue 

 colouring matter are not at present investigated from this 

 point of view. There is evidence, however, that light is 

 essential to the production of all varieties of Hippolyte, 

 except the reddish-brown ones. The other factor — the source 

 of pigment itself — is less known than are the conditions which 

 determine each particular tint. The colouring matter of the 

 food is one possible source, and this has to be briefly con- 

 sidered, since, if proved, it -would simplify the problem of 

 sympathetic coloration. That the sub-hypodermal colours of 

 caterpillars and beetle larvfe are due to diffusion of fatty 

 pigments from the food- contents of the gut is a conclusion 

 reached both by Poulton and Towers, though the physio- 

 logical details of this remarkable process have never been 

 ascertained. But thehypodermal coloui-s of these animals are 

 of an entirely different nature from those of Hippolyte, 

 and appear to be determined by enzymes, elaborated by this 

 layer acting upon the " primary " cuticle or retained within 

 tlie hypodermal cells. In Hippolyte and in Crustacea 

 o-enerally (as in the insect larvas), the first formed pigments 

 are developed independently of the plant-food present in the 



