144 M. A.-E. MalarJ on the Infuence of Lujht on 



an actual consequence of tlie insufficiency of liglit in the 

 medium in which they liavc lived. 



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

 tlie coloration of crustaceans is enormous, and we may say 

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

 class have a tendency to vary tlie coloration of their bodies 

 according to tlie coloration of the surrounding medium. 



Mcireover this chromatic adaptation of the animal to the 

 ambient medium seems to be effected in two dift'erent 

 fashions : — 



1, Bi/ chemical means^ that is to say, by the modification of 

 a ])ignicnt under the direct influence of light. 



2. Bii p/ii/t>wIo(/ical means, that is, by the action of pig- 

 nient-cells or chromatoblasts working under the influence of 

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

 ]'rocess which actually originates from the eyes of the 

 animal. 



Chromatic adaptation is met with in many Copepods, 

 An>phipods, Isopods, and Macrura ; it a})pears to be of less 

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

 placed by another form of mimicry. 



As long ago as 1S63 Claus mentioned the changes of colour 

 which are undergone by Copejwds belonging to the genus 

 Sajqihirina. According to this author, in the Copepods of 

 this genus the shield can assume changing colours, which the 

 animal varies according to the media in which it hapjjens to 

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

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

 ] ortion of the others the male alone is endowed with this 

 liower *. In 1867 an analogous statement was made by sSars 

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

 lastly, at a more recent date tlie same phenomenon was 

 obscivcd by llcrrick in the case of Diajttomus castor \. 



I he changes of colour in Squt'Ila and JJi/sis, under the 

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

 they have fornied the subject of recent papers by Weber § 

 and Schmidlcinli, and I 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-wccd which distiu- 



* Claus, ' Die freilebenden Copcpoden,' 1863, p. 35. 

 t G. (J. Saif , ' Histoire Naturelle des Cru8tac<58 d'Eau douce de Nor- 

 vege,' 18C.7, p. 23. 



\ Hinick, 'American Naturalist,' \ol. xvii. p. 381. 

 § Weber, Arcliiv f. ruikr. Anat. Jkl. xix. pp. OiJl, 6U7. 



II ifcbujidleiii, Mittbeilungeu zuul. Stal. ISeajiel, Dd. i., 1^70, p. oI3. 



