§ 3.223 EFFECTORS WITH MOVABLE PIGMENT GRANULES 83 



difficult to see (Plate 3-1) when their pigment granules "con- 

 centrate" at the centre. The branches reappear as the pigment 

 ''disperses" to their extreme tips. The cause of this pigment 

 migration has already been referred to (§ 3.22). 



The cells are sometimes named according to their pigments: 

 "erythrophores" for those with red pigment; "guanophores" with 

 white guanin; "melanophores" with black melanin, and "xantho- 

 phores" with yellow pigment. Some cells have black pigments 

 other than melanin. All these colours are to be found in vertebrates ; 

 the Crustacea, especially the Malacostraca, have other lipophores, 

 or coloured cells, with a blue pigment as well. Both may have 

 iridosomes, with a movable reflecting material giving a blueish- 

 white appearance (Fig. 3-18). 



The control of chromatophores in Crustacea is by hormones 

 only, but can be either by nerves or hormones or a combination of 

 the two in vertebrates. The type of control affects the response 

 somewhat, since hormones, which reach all the chromatophores 

 through the blood, tend to produce a slow and similar response in 

 all parts of the animal, whereas nerve control can quickly stimulate 

 individual chromatophores to produce a colour pattern that may 

 match the background closely, as in Pleuronectidae and cham- 

 eleons. Such an effect can only be simulated in those Crustacea in 

 which the chromatophores themselves are much differentiated and 

 each type responds to distinct hormones. This seems to be the case 

 in the prawn, Leander, in which some chromatophores are large 

 and together form dark bands, while others supply a stippled and 

 variable body colour. They include as least four colours and can 

 be adapted to a variety of backgrounds (Plate 3/1), but their 

 control is not yet fully elucidated (Knowles and Carlisle, 1956), 

 and is too complicated to use as an example here. 



Among vertebrates, it is the chromatophores of all Agnatha, 

 Elasmobranchii and Amphibia, but of only some species of 

 Teleostii and Reptilia, that are under hormone control (Fig. 3-24). 



The chromatophores of the leeches, Hirudinea, concentrate in 

 light and disperse in the dark, but show no background response. 

 They are probably all under nerve control, and need not be con- 

 sidered here. 



A typical background (or albedo) response is related to the 



