144 M, A.-E. Malard on the Infuence of Light on 



an actual consequence of the insufficiency of light in the 

 medium in which they have lived. 



Far from stopping here, indeed, the influence of light on 

 the coloration of crustaceans is enormous, and we may say 

 that, as an almost general rule, the animals belonging to this 

 class have a tendency to vary the coloration of their bodies 

 according to the coloration of the surrounding medium. 



Moreover tliis chromatic adaptation of the animal to the 

 ambient medium seems to be effected in two different 

 fashions : — 



1 . By chemical means, that is to say, by the modification of 

 a pignieiit under the direct influence of light. 



2. By physiological means, that is, by the action of pig- 

 ment-cells or chromatoblasts working under the influence of 

 light, but indirectly, and by the intervention of a sort of reflex 

 process which actually originates from the eyes of the 

 animal. 



Chromatic adaptation is met with in many Copepods, 

 Amphipods, Isopods, and Macrura ; it appears to be of less 

 common occurrence in the Brachyura, where it is often re- 

 placed by another form of mimicry. 



As long ago as 1S6.3 Claus mentioned the changes of colour 

 which are undergone by Copepods belonging to the genus 

 Sap2>hirina. According to this author, in tlie Co])epods of 

 this genus the shield can asieume changing colours, which the 

 animal varies according to the media in wliich it happens to 

 be. One species alone forms an exception to tlie rule and 

 seems to be devoid of this power of mimicry. In the greater 

 portion of the others the male alone is endowed with this 

 power *. In 1867 an analogous statement was made by Sars 

 with regard to Copepods of the Norwegian coasts f. And, 

 lastly, at a more recent date the same phenomenon was 

 obseived by llerrick in the case of DiajAomus castor \. 



The changes of colour in Squilla and Alysis, under the 

 influence of light, have been known for a very long time; 

 they have formed the subject of recent papers by Weber § 

 and Schmidleinll, and 1 merely mention them here in passing. 



Among the Isopods the phenomenon is most interesting 

 and easiest to determine in the genus Idotea. 



In the line of foam and floating sea-weed which distin- 



* Claus, ' Die freilebeudeu Copepoden,' 1863, p. 35. 

 t G. O. Sars, ' Histoire Naturelle des Crustac^s d'Eau douce de Nor- 

 vege,' 1807, p. 23. 



X Herrick, 'American Naturalist,' vol. xvii. p. 381. 



§ Weber, Archiv f. mikr. Aiiat. Bd. xix. pp. 691, 597. 



II Sclunidleiu, MittheiluDgeii zoul. Stat. Neapel, ISd. i., 1879, p. 513. 



