§ 3.224 EFFECTORS WITH MOVABLE PIGMENT GRANULES 109 



the same chromatophore, unlike the complex pattern-forming 

 chromatophores of Leander and Penaeiis (Knowles, 1955 ; Knowles, 

 Carlisle and Dupont-Raabe, 1955, and Knowles and Carlisle, 

 1956), in which there may be as many as four. It is to be hoped 

 that the refined technique developed by these authors for separa- 

 ting pure substances from tissue extracts by paper electrophoresis 

 will soon be extended to the circulating blood of these and other 

 prawns, with their chromatophores in different states of dispersion 

 in response to different states of illumination or background 

 colour. When extracts of tissues yield the same substance as that 

 found to be active in the blood, it should be possible to identify 

 the sources of hormones actually used by the animal, and to 

 distinguish these from other substances, like acetylcholine, pro- 

 duced at nerve endings in the central nervous system ; for these can 

 react upon effector systems experimentally, and yet never reach 

 them through the circulation in the living animal (Knowles, 1955). 

 Modern techniques might also throw light upon the rate of 

 chromatophore reaction, which seems normally to be so much 

 slower in the vertebrates than in the Crustacea, and to be controlled 

 to a considerable extent in the latter by the concentration of the 

 hormone reaching the cells. The rates could be compared with the 

 high speed of reaction of nerve-controlled chromatophores, to see 

 if the differences were due to the more concentrated dose of the 

 chemical which can be supplied at a nerve ending, rather than to 

 differences in sensitivity of the chromatophores (Waring, 1942). 

 Nevertheless, Waring's idea, that an evolution in either the 

 sensitivity of the chromatophore or in its speed of reaction was a 

 necessary precursor of the evolution of nerve control with adaptive 

 value, is interesting. It appears to be borne out to some extent by 

 his table showing that nerve control is only of importance in 

 vertebrates of more recently evolved families, and has not been 

 achieved in the Elasmobranchii and Amphibia, which are both 

 classes with a much longer fossil history than either the Teleostei 

 or the Reptilia (Fig. 3-24). The persistence of purely hormonal 

 control in Crustacea would then be expected, and could be looked 

 upon as having evolved along lines of chromatophore differenti- 

 ation with multiple hormone control, instead of being replaced 

 by nerve control. 



