§ 3.223 EFFECTORS WITH MOVABLE PIGMENT GRANULES 95 



Palaemonetes. The white chromatophores (Table 10) reinforce 

 the protective colour change provided by the red pigments of this 

 prav^n (Brown, 1950a). The concentration of white pigment, 

 which occurs naturally on a dark background, can be induced by 

 injecting an extract from the commissures. This contains a 

 Pfl/^^mow^fg^-w^HiTE-coNCENTRATiNG HORMONE, PWCH. The dis- 

 persion, that occurs on a white background, can be induced by a 

 Pa/«^mO«^^^5- WHITE-DISPERSING HORMONE, PWDH, from the 

 sinus glands. 



The direct effect of light on the skin is to cause pigment disper- 

 sion in these white chromatophores, as it does in erythrophores ; 

 but here it reinforces the hormone reaction controlled by the eyes, 

 instead of counteracting it (Tables 10 and 11). 



Crago*, The hormonal control of white chromatophores in the 

 shrimp follows the same pattern as that in Palaemonetes. The 

 sources of the two hormones, CWCH and CWDH, are probably 

 similar, but this is not certain. There appears to be no direct effect 

 of light upon the guanophores in the skin, for they remain con- 

 centrated, whatever the light intensity. 



Uca. In Brachyura, the control of the white chromatophores, 

 like that of the red and black cells of the same animals, follows a 

 pattern distinct from that of the other Decapoda. Although the 

 background response is the same as in the prawns, the source of 

 the L^cfl-WHITE-DISPERSING HORMONE, UWDH, Cannot be mainly 

 in the sinus gland, as theirs is, since the pigment remains perma- 

 nently dispersed in eyestalkless animals. The latter observation 

 suggests that, like the f/^a-red-dispersing hormone, the source of 

 the C/Cfl-WHITE-CONCENTRATING HORMONE, UWCH, might be in 

 the sinus gland; but extracts have not so far given any concen- 

 trating effect. There is apparently no direct effect of light upon 

 the white chromatophores of Ucay which remain permanently 

 dispersed, even in the dark, in the absence of hormonal control. 



Teleostei. The white chromatophores, or guanophores, of the 

 killifish, Fundulus, show the same adaptive reaction to background 

 colour as do those of the Crustacea, dispersing on a white back- 

 ground and concentrating on black (Fig. 3-18 a and h). Their 



*This spelling of Crangon is commonly used in this context in 

 America, and is retained here for simplicity. 



