40 COMPARATIVE PHYSIOLOGY 



thoroughly investigated, the chromatophores are highly complex 

 structures of a multicellular or coenocytic character. However, 

 the structure of the Crustacean chromatophore differs very 

 widely in the different groups and in different regions of the 

 same individual. A necessary prerequisite to quantitative 

 treatment in this case is a clear recognition of the normal 

 behaviour of the pigmentary effector organs. The phenomena 

 of colour response in Crustacea may be illustrated by reference 

 to the Schizopod Macromysis and the Decapod Hippolyte, as 

 described by Gamble and Keeble (i 900-1 906). 



In Macromysis the chromatophores are localised in definite 

 regions lying for the most part in a deep situation where their 

 action is rendered visible by the translucent nature of the 

 integument. The majority of the chromatophores contain a 

 large amount of dark brown pigment and a smaller quantity 

 of a substance which by reflected light has a white or yellow 

 hue, but appears greyish by transmitted Hght. On a sandy 

 shore the animal appears transparent and colourless or greyish 

 in tint ; amid dark surroundings and in deeper water, its 

 colour deepens and a pattern becomes manifest in the form of 

 paired arborisations of yellowish-white upon a brown back- 

 ground corresponding to the chromatophore centres. In the 

 transparent, colourless, or greyish form the brown and yellow 

 pigment is withdrawn into the central body of the chromato- 

 phores ; in the dark condition the brown and yellow pigments 

 diffuse along separate paths throughout the interlacing tracery 

 of branches ; the yellow pigment does not mix with the brown, 

 and it responds to stimuli at a different rate. 



In Hippolyte we have to draw a distinction between two 

 modes of colour response occurring normally in nature. By 

 day the adult prawns are of a reddish- brown or bright green 

 hue according to the tint of the seaweeds to which they are 

 attached. Whatever the diurnal colour of Hippolyte may be, 

 however, it changes at or soon after nightfall to a beautifully 

 transparent blue or greenish-blue. The depth of the nocturnal 

 blue corresponds to the intensity of the diurnal colour ; that 

 is to say, dark brown prawns become deep blue and light ones 

 become pale blue. The nocturnal tint ordinarily persists till 



