HIPPOLYTE VARIANS. 643 



declares itself when the external stimulus — change of light- 

 intensity — is cut off. The '^dark-induced" nocturnes are very 

 susceptible to light-stimuli, the " light-induced " nocturnes 

 are on the contrary refractory to them. The periodicity is worn 

 down by constant light-conditions. Though we have evidence 

 that the final phase of long-continued exposure to constant 

 conditions of darkness is one nearer the nocturnal than the 

 diurnal state^, we are not prepared to state that this is the 

 case. Blinded prawns exhibit periodicity, though often 

 the phenomenon is complicated by the immediate effects of 

 the operation. The shock does not appear to wear off com- 

 pletely, since prawns, whose eyes have been rendered func- 

 tionless by section of both optic stalks, are longer in 

 performing the complete cycle of changes. 



The possibility of permanent nocturnes and of " colour 

 rigor" must be borne in mind in investigations on the 

 colour of deep-sea Crustacea. 



Periodicity, which we believe is a new fact in colour 

 physiology, may turn out to be a phenomenon of very wide 

 occurrence among animals with contractile " chromato- 

 phores." 



Section VII. 



The Control of the "Chromatophores : " the Parts 

 played by the Eye, Central Nervous System, and 

 by the Chromatophores themselves, in effecting 

 Colour-change. 



The fact that Hippolyte varians is at any moment of 

 the day or night passing towards or away from the nocturnal 

 condition must be reckoned with in any attempt to explain 

 alteration of the chromatophoric condition. In discussing, 

 in the last two sections, the effects of external agents such as 

 differently coloured seaweeds and varying light-intensities we 

 pointed this out. We have now to keep it in mind whilst 

 endeavouring to trace the nature and the course of the 

 impulses which play upon the "chromatophores" themselves. 



That the "chromatophores" respond independently of the 



