HIPPOLYTE VARIANS. 649 



While changes in the quality and intensity of light act no 

 doubt to a large extent through the eyes^ other of these 

 agents must have acted independently of these organs^ and 

 appear to influence the central nervous system directly, and 

 through it the chromatophores. We have, however, some 

 reason for believing that a redistribution of the pigments in 

 these bodies may take place independently of nervous 

 control. Apart from the periodically varying action of the 

 nervous system, it seems probable that any condition of the 

 chromatophores is largely maintained by impulses passing to 

 them from the nervous system. The impulses passing from 

 the optic ganglia to the " brain " cause repeated alteration of 

 those received from the ventral ganglia. When, for example, 

 by amputation of the eye the optic impulses are cut off, 

 profound changes occur often immediately ; changes which 

 result in a retraction of red pigment, and even in extrusion 

 of the blue. Recovery subvenes. The central nervous system 

 would appear to reassert itself and reassume its presiding 

 influence over the chromatophores. 



Two considerations only remain: how can the phenomena 

 of periodicity and of the nocturnal phase be interpreted in 

 terms of utility ? and of what practical importance are they in 

 further studies of the colour-changes of Crustacea ? On the 

 first point we prefer to say nothing till we have learnt more 

 of the habits of Hippolyte varians and of its nearest 

 allies. We require to know, for example, whether periodicity 

 is acquired during a lifetime, or is transmitted as a quality 

 of the bodily structure of the larva; whether any of these 

 prawns are or were naturally pelagic during the whole 

 or part of the day ; whether, in the particular instance of 

 Hippolyte varians, the change from the free-swimming 

 larva to the quasi-sessile life of the adult is accompanied by 

 the appearance or modification of these phenomena. It is 

 conceivable that the " nocturne " is a nightly reversion to 

 the blue-green colour so characteristic of pelagic animals ; 

 that the whole purpose of the wonderfully elaborate chi'o- 

 matophoric system is to enable the prawn to respond to 



