CILIARY ACTIVITY 41 



daybreak, when the green and brown varieties represent inter- 

 changeable states. Green Hippolytes placed on brown weeds 

 may conserve the green colour for about a week before assuming 

 the brown condition. The extent and rapidity of the change 

 varies very much from one individual to another. There are 

 three chromatophore pigments involved : red, yellow, and 

 blue. In addition to the more deep-seated elements in the 

 nerve cord, viscera, and principal muscles, the epidermis is 

 itself in Decapod Crustacea richly charged with chromato- 

 phores. These consist of groups of as many as eight pear- 

 shaped cells with tubular branches, each containing pigment of 

 one colour, yellow or red. Blue pigment is present in the 

 tracts along which the red pigment flows, but is also found in 

 branched cells lying among the pear-shaped cells which contain 

 the yellow or red pigments. The reddish-brown tones are asso- 

 ciated with the extension of the red pigment into the tubular 

 processes of the chromatophores. In the green condition the 

 yellow pigment extends throughout the branches of the cells in 

 which it occurs, but the red pigment is retracted, its place being 

 occupied by the blue substance. Finally, the nocturnal blue 

 colour is produced, when both yellow and red pigments are 

 withdrawn from the branches, but the blue substance diffuses 

 along them. 



The bulk of recorded observations on Crustacean chromato- 

 phores are concerned with the more inmiediate problem of 

 analysing what forces incident to the animal's environment 

 promote colour response, and the channels through which the 

 different stimuli gain access. Both Crustacean and Piscine 

 chromatophores, however, offer admirable material for studying 

 the physical chemistry of the cell. 



Whereas in Fishes and Amphibia, though possibly not in 

 Reptiles, light and dark backgrounds appear to act merely by 

 modifying the intensity of illumination, in the case of Crustacea 

 it would seem from Gamble and Keeble's researches that 

 something besides intensity is involved. To detail the chief 

 colour phases induced by various light conditions in Macromysis 

 and Hippolyte, it will be necessary to call attention to a 

 difference between the two genera. Freshly- caught Mysids 



