Ii8 Influence of Light on the Coloration of Crustaceans. 



The cliange of colour from the pale to the dark condition 

 occupies but a h\\ minutes ; it is different with regard to the 

 oj^posite process. During the disappearance of the deep 

 reddish-brown tint to become pale yellow and almost trans- 

 parent the observer noticed that the animal passed through an 

 intense deep blue stage. 



M. Pouchet has furnished the explanation of these pheno- 

 mena* bv showing how the action of two kinds of pigments 

 took place in them ; on the one hand the pigments of the 

 xanthic series (red, orange, and yellow) by the action of the 

 chromatoblasts, on the other the pigments of the cyanic series 

 generally free and in solution. The removal of the eyes 

 produces in Pnlcrmon the same effect as a black bottom f- 



In certain Brachyurous Crustaceans we again meet with 

 facts of the same kind ; but here they are less general and 

 not so numerous. Fritz Muller \ mentions an instance in a 

 Brazilian species of Gelasimus which is of a uniform greyisli- 

 brown colour in the female. In the male of this Gelasimus at 

 the breeding- season the posterior portion of the cephalothorax 

 is of a pure white, while the anterior region assumes a rich 

 green colour, passing into dark brown ; in the event of danger 

 and on the animal being alarmed its colours are subject to 

 moditication in a few minutes, the white becoming dirty grey 

 or even black and the green losing all its brilliancy. Not 

 until we come to Carcinus mcenas do we find that similar facts 

 have been reported ; I have myself often observed that the 

 crabs living upon a bottom clothed with Ulva, as at the mouth 

 of the Serre, near St.-Vaast, for example, when angry have a 

 more decidedly green tint on the dorsum of the cephalothorax 

 than those which are met with among the stony bottoms of 

 the old oyster-beds and in the Laviinaria-zowQ, where they 

 assume olivaceous hues, passing into dirty yellow and brownish 

 red, a shade which is in perfect agreement with the general 

 tone of the bottom. Messrs. Carrington and Lovett §, in 

 recording analogous observations, state that they have been 

 able to study the mechanism of the phenomena in the tanks 

 of the Westminster aquarium ; here again the facts observed 

 are probably due to the action of chromatoblasts. 



1 do not know whether the chromatoblasts also play a 

 similar part in the Mediterranean Lambrus, the curious 



* C. R. 1878, t. Ixxxvii. pp. 302-30.3. 



t M. S. Jourdain has since shown that by removing the eyes and. 

 leaving the animal in the dark a red coloration is always obtained. 



X According to Darwin, ' The Descent of Man,' French edition, p. 361 

 [2nd English ed., 1883, p. 271.] 



§ Carrington and Lovett, ' Zoologist,' 18S2, pp. 12 and 14. 



