444 CRUSTACEA MALACOSTRACA. 



ficial in position, and in the adult Decapod completely masking 

 the primary ; though this, none the less, persists. 



When the chromatophore pigment is expanded, a network of 

 colour, close or open, pervades the body, either continuously or in 

 certain parts, and then forming a pattern which in most species 

 is constant and characteristic. When it is contracted the 

 pigment forms an inconspicuous dot in the chrornatophore, 

 and the body of the Crustacean becomes transparent and 

 apparently colourless, though in some cases a diffused blue colour 

 remains. 



The movements of the pigment occur in response to light, 

 the action being direct and indirect. (1) The direct action of 

 the light is to cause expansion of the pigment. In the dark it 

 contracts and the degree of expansion varies with the intensity 

 of the illumination. (2) The indirect action of light is effected 

 by the nervous system through the mediation of the eyes. The 

 response is not to the intensity of the illumination, but to the 

 colour of the background against which the prawn finds itself. 

 A dark background causes expansion of the pigment, in Macro- 

 mysis, Palaemon and Hippolyte, even though the light be dim ; 

 a light background causes contraction. At night (absence of 

 direct stimulus of light) the pigments contract, the animals 

 becoming transparent. The colour pattern develops again at 

 dawn. 



An astonishing variety of colour coats is at the disposal of 

 Hippolyte variant,, enabling the individuals of this species to 

 blend in colour with their surroundings. They are of very 

 sedentary habit and only quit hold of the weed among which 

 they live when it is roughly shaken. They may be found of 

 uniform brown, red, pink, grey or emerald green colour, or 

 broad bands or blotches or thin lines of these colours may traverse 

 the otherwise transparent body. The pattern, uniform or 

 localized, is found to correspond with the degree of subdivision of 

 stems or branches of the weed which the prawns inhabit. When 

 removed from their natural habitat and given a choice of weed 

 they take refuge in that which they most closely resemble. 

 By prolonged residence (some days) among weed of a different 

 colour, the colour of the coat pattern of the prawn may vary in 

 correspondence e.g., brown becoming green or vice versa. 



Although the colour is thus adaptable to that of the environ- 



