The Colour-Physiology of Hippolyte varians. 



463 



(b) During microscopic examination, grave colour-changes often 

 occur j yet with practice, a very rapid examination may be made, and 

 so the source of error considerably reduced. The colours to be re- 

 corded are often several, the gross colour of the animal being seen 

 under the microscope to be due to several pigments (see below Sect. III). 

 These pigments are differently distributed in different colour-forms, so 

 that the pigmentary records become rather complicated. Any but the 

 briefest microscopical examination throws the nervous system of the 

 animal out of gear, produces after-effects, and too frequently renders 

 the animal useless for further experiment. Control-specimens must 

 be used before conclusions can be drawn from the simplest experiments, 

 and experiments must be confirmed several times. Added to these 

 difficulties is this, that colour-change in Hippolyte is no simple reflex 

 affair taking place " with the certainty of a physical experiment,'*' but 

 is one subject to what, in times of difficulty, seems to amount to wilful 

 perversity. "What we believe to be the chief element in this seeming 

 perversity is described under the head of Periodicity in Section VI. 



III. The Nature of the " Chromatophores." 



The colours of the pigments in the " chromatophores " determine 

 the tint of the prawn by their disposition and the depth of its colour 

 by their abundance. The "chromatophores" are by no means the 

 simple, stellate, cellular, dermal structures which they are commonly 

 supposed to be. One series of them lies under the epidermis, another 

 is interspersed between the muscle-fibres both of the great flexors and 

 extensors of the tail and those of the appendages, while a third series 

 — often forming great splashes of colour — invests the gut, nerve-cord, 

 liver, and other internal organs. 



In simple colour varieties — brown for instance — the pigment of the 

 skin forms a dense network obscuring the " muscle-chromatophores." 

 In such cases the colour of the prawn is determined by the colour of 

 the superficial network. In other cases, when the prawns are banded 

 or boldly barred, the colour-elements of the skin are absent or have no 

 pigment, and the deep " chromatophores " alone determine the colour 

 of the pattern. In many Hippolyte we have found that distribution 

 of the pigment is the same in the skin-chromatophores and in the 

 muscle-fibres which underlie them. The two sets are co-ordinated. 

 This correspondence applies to other Crustacea, though it has not 

 hitherto been recognised. 



The pigments present in the chromatophores are limited to red, 

 red and yellow, or red, yellow, and blue. These three may be present 

 together in one and the same element. During colour-changes they 

 are distributed independently of one another in the sense that one 

 pigment may become aggregated in the centre of the " chromatophore," 



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