650 F. W. GAMBLE AND F. W. KERBLE. 



changing intensities of liglit; that the nocturnal phase of 

 colour (or colourlessness) is an extreme expression of the 

 contractility of this system — is a defect of its qualities; and 

 that the daily life of the prawns' colour-work is "rounded 

 with a sleep." 



The other question, of the practical importance of pe- 

 riodicity, is one which admits of a more confident statement. 

 It is clear that this phenomenon has to be borne in mind in 

 the description of any colour at all, and especially of any 

 colour-change induced by stimulus ; and that since the 

 colour-condition of the animal is a function of the time of 

 day, that time of day must be taken into account before 

 conclusions can be drawn as to the real effect of the stimulus 

 in question. 



(An abstract of this paper has appeared under the title 

 "The Colour-physiology of Hippolyte varians" in the 

 ' Proceedings of the Eoyal Society,' January, 1900, vol. xlv.) 



