§ 3.223 EFFECTORS WITH MOVABLE PIGMENT GRANULES 93 



effects an initial response which is practically that which would be 

 achieved in the complete absence of the second. Such a mechanism 

 would assure rapid response in either direction, provided the time 

 elapsing between the responses is adequate for the normal reduc- 

 tion in titre after the initial secretory burst" — i.e. about 15-50 

 min in the black-to-white change, and 30-60 min in white-to- 

 black (Brown, Webb and Sandeen, 1952). 



Eupagurus. The red pigment in the hermit crab, Eupaounis, 

 (Table 11) behaves like that in Palaemonetes ', it is concentrated in 

 bright light on a pale background, and dispersed on a dark back- 

 ground. It is clear that both reactions in Eupagurus are under 

 hormone control, but the actual substances and their exact sources 

 have not been determined, although removal of the eyestalks 

 again results in persistent darkening. 



Uca. The background responses of the red pigment in the 

 chromatophores of the fiddler crab, UcUy show well and have 

 already been referred to (Fig. 3-lSa). Both red-concentrating and 

 -dispersing substances can be extracted from nearly all parts of 

 the nervous system; but, as in most brachyuran crabs, the sinus 

 gland is the richest source of t7c«-RED-DiSPERSiNG hormone, 

 URDH (Fig. 3-15^ and d). As the same is true of the hormone 

 dispersing the melanophores, removal of the eyestalks of crabs 

 results in permanent pallor (Brown, 1950^). This is unlike the 

 situation in prawns and most other Malacostraca that have been 

 examined, where the sinus gland provides a concentrating hor- 

 mone, like PLH, and removal of the eyestalks therefore results in 

 permanent darkening. The main source of the Uca-BED-CONCEN- 

 TRATING HORMONE, URCH, is in the circumoesophageal connec- 

 tives (Fig. 3-1 5c) and this is again the opposite site from that in the 

 prawns (Tables 9 and 11). The sinus gland also yields small 

 quantities of URCH. 



On a constant white background Uca also shows a diurnal 

 rhythm of colour changes that are comparable to the direct effects 

 of light and darkness on most dark chromatophores. The crabs 

 become dark by extensive pigment dispersal by day, and pale at 

 night; but these changes in Uca are not merely direct effects 

 (p. 84), since their rhythm persists for four days in constant dark- 

 ness, and it is not altered, although the amplitude of the pigment 



