HIPPOLYTE VAEIANS. 613 



mined by the mode of distribution of the '' chromatophores/' 

 and by the relative degrees of expansion of their contained 

 pigments. We have described some of the more prevalent 

 colour forms, without more than incidental reference to their 

 possibilities of colour-change. 



That Hippolyte varians is to some extent capable of 

 colour-change is well known (see Kinahan, 1857 ; Herdman, 

 1892, etc.), so that what we have described as different colour 

 forms are in reality but the most stable phases in animals 

 undergoing a series of colour-changes. We have now to dis- 

 cover the extent of these changes in each case, the conditions 

 under which they occur, and in what way change in condi- 

 tions affects the redistribution or other modifications of the 

 pigments of the '^ chromatophores." 



Earlier observers have attributed change of colour to change 

 in the nature of the ground over which the animal is passing, 

 to its change of habitat from one coloured weed to another, 

 and also to a limited extent, in captivity (Pouchet, Malard), 

 to change in illumination. 



When we remember that such classical problems as the 

 causes of colour-change in the frog and the chameleon have 

 only been quite recently thoroughly reinvestigated — that of 

 the frog by Professor Biedermann (1892), of the chameleon by 

 Dr. Keller (1895) — we shall not be surprised that uncertainty 

 should still exist as to the nature of the stimuli which modify 

 the coloration of these prawns. Professors Biedermann and 

 Keller have shown that colour- change in the frog and the 

 chameleon is produced not by light only, but also by contact 

 of the toes with different substrata, by variations in tem- 

 perature, and by changes in the amount of oxygen available 

 for tissue respiration. These external changes are all 

 effective in exerting a certain nervous activity which is mani- 

 fested by change of colour. We are still engaged in 

 endeavouring to unravel the tangled skein of causes in the 

 case of colour-change of Hippolyte; as yet the only factor 

 the influence of which on colour-change we have investigated 

 with any fulness is light. In this section, therefore, we shall 



