108 KINETIC HORMONES — I 



pigmentary effectors with movable granules, including some which 

 have nerve control as well. But in nearly all cases the actions and 

 sources of one hormone are much more fully established, than 

 those of the other; the more certain of the two is the light-adapting 

 hormone for Crustacea, and the fishes Phoxiniis and Fundiilus, 

 whereas it is the dark-adapting hormone for the Amphibia, 

 Elasmobranchii and some Teleostei, such as Ameiurus. It seems 

 plausible to expect that in the stick insect, where so far only the 

 Car flw^/w^- darkening hormone has been found (§ 3.221), an 

 antagonistic pigment-concentrating hormone wdll be revealed in 

 due course. 



In Crustacea there is evidence from Isopoda and from many 

 Decapoda, other than Brachyura, that the hormones producing 

 the light background response are normally stored in, and pre- 

 sumably discharged from, the sinus gland or its equivalent; 

 this is true for the concentration of dark pigments such as black 

 and red, and also for the converse dispersion of the white pigment 

 (Tables 10, 11). Similar converse reactions of the pigment in 

 coloured and white chromatophores in fish may also be due to 

 one and the same hormone, as in Fundulus. 



The situation in the Brachyura is peculiar. There is a background 

 response, which though slight is similar to that in prawns; but, 

 instead of the concentrating hormone, it is the dispersing hormone 

 which is released from the sinus gland in the eyestalk. In many 

 crabs, such as Uca, the background response is overridden by an 

 opposing reaction causing darkening in bright light. It is tempting 

 to interpret this as being either (i) a morphological accident the 

 result of which is that, when light stimulates the sinus gland via 

 the eye, the gland releases its hormone, as in other decapods (but 

 this produces the opposite effect in crabs because the secretion is 

 a dispersing instead of a concentrating hormone); or (ii) the effect 

 of a single hormone controlling dispersion of pigment in both the 

 melanophores and the retinal cells; for dispersion of retinal 

 pigment, as in Cambariis, is the adaptive response to light and the 

 daytime dispersion of the crabs' melanophores would then be but 

 following in the wake of their retinal pigment. 



It must be emphasized, however, that none of the cases consid- 

 ered (except Crago) have more than one colour of pigment in 



