616 F. W. GAMBLE AND P. W. KEEBLE. 



colour. On microscopic examination the red pigment, which 

 had before been retracted, was now fully expanded, the yellow 

 more retracted. The other specimen was dull green, but had 

 emitted some red pigment. The change was in the direction 

 of a red tint. The failure to assume it may be connected with 

 the fact that this red colour is associated with an absence 

 of yellow, whilst the green prawn possesses much pigment of 

 this colour. 



The four experiments which may be taken as typical of a 

 large number (pp. 658 — 669) show that a certain sympathetic 

 change of colour is possible, but that the range of colour muta- 

 tion accompanying change in the colour of the weed is not 

 great, and that the rate of change is generally very slow. With 

 some forms, notably the small Hippolyte varians, which 

 are always of a bright green tint when living in shallow 

 water amongst the leaf -like branches of Zostera, little or 

 no colour-change attends their transference to weed of 

 another colour. Indeed, in a batch of prawns of any colour 

 there are always a considerable proportion which are un- 

 affected by such changes. 



We must now return to the first of the three experiments 

 just given, to consider the apparently astonishing result that 

 a sympathetic change of colour from brown to green takes 

 place in the dark. 



In endeavouring to account for this sympathetic change 

 occurring in the dark, two facts must be borne in mind, for 

 in all probability at least two factors are involved. In the 

 first place, light is not the only efiicient stimulus to colour- 

 change, as we demonstrate in Section VII ; and in the second 

 place the greening of the brown Hippolyte is attributable, 

 at least in part, to an after effect of the light-stimulus. 



During the night the particular trend of nervous activity 

 which had determined the diurnal colour, in combination 

 with external conditions, is suspended. The animal during 

 the night assumes an entirely different hue, of which we 

 speak later. On the succeeding day, even though light be 

 excluded, the animal resumes its nervous way, takes up its 



