§ 3.223 EFFECTORS WITH MOVABLE PIGMENT GRANULES 99 



by diurnal changes, which operate in the opposite sense. If, 

 however, allowance is made for the diurnal changes, it can just he 

 seen that, for any given intensity of light incident upon the 

 background, the degree of dispersion of the black pigment is 

 greater on a black background than on a white background 

 (Brown and Sandeen, 1948). The Uca-DARKENiNG hormone, 

 UDH, controlling dispersion of the melanophore pigment, like 

 the Uca-red-dispevsing hormone, URDH, is secreted from the 

 sinus gland; eyestalk removal therefore results in relative pallor. 

 The dispersing activity of UDH can be neatly demonstrated by 

 using the two sexes of these crabs (Fingerman and Fitzpatrick, 

 1956). Melanophores in the female are normally more dispersed 

 than in males in the same situation. If the large, hollow, asym- 

 metrical chela, which distinguishes the male from the female, is 

 removed before the crabs are exposed to light, the operated male 

 becomes as dark as the female. If more legs are removed he be- 

 comes even darker. This can best be interpreted by assuming that 

 the same amount of dispersing hormone, UDH, is released in each 

 animal in response to similar stimuli ; but that the degree of disper- 

 sion of the chromatophores depends upon the concentration of the 

 hormone in the blood. This is increased as the blood volume is 

 decreased by removing successive appendages. 



The natural secretion of [/crt-LiGHTENiNG hormone, ULH, has 

 been demonstrated by using the blood of crabs, in which the 

 melanophores were maximally concentrated, to perfuse isolated 

 limbs on which the melanophores were dispersed. These show 

 slow pigment concentration even if perfused with sea water ; but 

 the rate of concentration is increased if perfused with blood 

 containing ULH (Fingerman, 1956). The source of ULH has not 

 been located ; but, unlike CTLH, it cannot be in the eyestalk. 



Ltgia. Apart from the Decapoda so far described, the only 

 crustacean that has had its melanophores investigated in detail is 

 the sea slater, Ltgia, among the Isopoda. In nature it changes 

 colour from black, when lurking in damp and shady crevices, to 

 pale mottled grey, when exposed to light. On any black background 

 in bright light they show the usual response, by which the melano- 

 phores become fully dispersed (index 5). On an experimental white 

 background the pigment can become concentrated to an unnatural 



