§3.223 EFFECTORS WITH MOVABLE PIGMENT GRANULES 85 



is due to antagonism between the direct dispersion and the second- 

 ary hormonal concentration can be seen by quickly removing the 

 crab from its sheher; at first, the newly exposed abdominal 

 chromatophores are concentrated by the hormone effect only, but 

 rapidly darken to match those on the limbs, as a result of exposure 

 to the direct effect of the light. A similar direct effect occurs 

 among reptiles. When a cUmbing lizard, AjioUs, is blinded, its 

 chromatophores show dispersal in the light and concentration in 

 darkness (Brown, 1950^). The degree of dispersion usually in- 

 creases with increase in the light intensity. 



Temperature increase can concentrate the pigment in the white 

 chromatophores of Palaemonetes (Fingerman and Tinkle, 1956) 

 and thereby counteract its dispersion in response to a bright white 

 background. This may help to keep the state of dispersion under 

 control in very bright, hot conditions. In reptiles, such as Phryno- 

 somaj concentration of dark pigments at high temperatures may 

 help to prevent excessive heat absorption by the body. 



Moisture disperses some pigments, like those of Caraiisius 

 (§ 3.221) and that in the melanophores of the frog, Rana, 

 where the concentration due to a light background may be 

 enhanced by dry conditions or almost completely overcome by total 

 immersion. Other degrees of moisture give intermediate results. 



A rhythm of colour change related to the tides is said to persist 

 for some time, under still water conditions in the laboratory, in inter- 

 tidal forms such as the fiddler crab, Uca pugnax. A diurnal rhythm 

 of colour change persists under constant conditions of either light 

 or darkness among other Crustacea, Insecta and Amphibia, and may 

 be mediated by hormones in some cases, such as Ucapugilator. 



In experiments designed to show the action of hormones in 

 controlling physiological colour change, care must be taken to 

 control or eliminate these extraneous factors, affecting the reac- 

 tions of the chromatophores. Once this has been done, two main 

 lines of attack on the problem can be followed, though a combina- 

 tion of the two is needed for its full elucidation. 



The first method is that due to Hogben and Slome (1931 and 

 1936), who studied the behaviour of the chromatophores in 

 relation to changing environmental factors in intact Xenopus. 

 This, which may be called the physiological method, they later 



