§ 3.223 EFFECTORS WITH MOVABLE PIGMENT GRANULES 97 



normal control may be by nerves, as is that of the melanophorcs of 

 this fish; but both types of chromatophores react rapidly to 

 injections of either adrenaline, or Antuitrin, that supplies a 



MELANOPHORE-CONCENTRATING HORMONE, MCH (Odiornc, 1933) 



to give the "light reaction" in which melanophorcs concentrate, 

 and guanophores disperse (Fig. 3-186). Although denervatcd 

 melanophorcs of Fundulus are known to disperse in response to 

 injections of B (or MSH), there is only slight evidence that this 

 causes the expected concentration of the guanophores. 



Black pigment in chromatophores 



Crustacea. The shrimp, Crago, is the only member of the 

 Decapoda Natantia so far investigated that has black pigment akin 

 to melanin in cells that may be called melanophorcs. These melano- 

 phorcs can produce the beginnings of a pattern, because they are 

 differentiated into two sizes, larger on the body and smaller on 

 the tail. These both react to one Cr«^o-DARKENiNG hormone, 

 CDH, from the commissures, but are concentrated independently 

 (Brown, 1946 and 1950«); those on the tail by Crago-TAiL- 

 LIGHTENING HORMONE, CTLH (alcohol-insolublc extract from 

 the SINUS gland), and on the body by Cr^^o-BODY-LiGHTENiNG 

 HORMONE, CBLH (water-soluble extract from the brain), both 

 of which are dominant to CDH. In nature, this means that the 

 shrimp can go wholly dark if only CDH is acting, or the tail or 

 body only may be lightened, while the rest remains dark, accord- 

 ing to which of the concentrating hormones are present with the 

 CDH. Finally, if both CTLH and CBLH are present, the shrimp 

 becomes completely pale. An eyestalkless specimen lacks CTLH, 

 and is at first pale with a dark tail, but after a time this effect dis- 

 appears. The natural stimulation of pattern forming changes in 

 the shrimp is not known ; but in prawns, like Leander, the exten- 

 sion of a similar system to a large number of different types of 

 chromatophores, apparently each controlled by separate hormones, 

 must give them adaptive advantages by increasing their ability to 

 match a variety of backgrounds (Plate 3-1). 



Uca. The background responses of Uca melanophorcs (Fig. 

 3-19) are more difficult to demonstrate than those of the red 

 chromatophores, because they are almost completely overridden 



