42 COMPARATIVE PHYSIOLOGY 



and some shrimps, e.g. Palaemon, generally have their pigments 

 moderately expanded ; and this represents the condition of 

 the chromatophores of the same animals when kept in the light 

 in glass containers. In Hippolyte, on the other hand, the 

 yellow and red pigments are always fully expanded in the day 

 and fully contracted at night. In darkness the yellow, red, and 

 brown pigments of both Hippolyte and Macromysis become 

 completely retracted into their chromatophore centres, which 

 then appear as minute dots so widely separated that they do not 

 interfere with the translucency of the body as a whole ; the 

 time required to produce the contraction ranges from a few 

 minutes to an hour or two, varying considerably with the time 

 of day and the condition of the animal. 



Paradoxically enough, the same result is brought about by 

 exposing Crustaceans to a pure white reflecting background, 

 such as a porcelain surface. When subjected to such treat- 

 ment there occurs a condition essentially the same as the 

 nocturnal state accompanied in Hippolyte by diffusion of the 

 blue pigment already referred to, and conditioned in both 

 Macromysis and Hippolyte by the complete retraction of the 

 other pigments into the chromatophore centres. This response 

 is exceedingly rapid, being accomplished in about a minute or 

 even less. The effect of a dull black, Hght-absorbing back- 

 ground is no less surprising. In Hippolyte itself it cannot be 

 demonstrated because the red and yellow pigments are, as 

 stated, always expanded fully in daylight ; but in Macromysis 

 and Palaemon, where expansion on a neutral background is 

 incomplete, the normal accompaniment of transferring indi- 

 viduals to dark-bottomed vessels is to induce a condition of more 

 extreme outward migration of the chromatophore pigments 

 from the chromatophore centres into the branching processes, 

 so that the animal assumes in the case of Macromysis nigra 

 a black aspect and in the case of Palaemon serratus a speckled 

 brown coloration. These colour phases persist so long as 

 the light conditions remain unchanged. At nightfall the 

 pigments which have expanded in the " dark background 

 phase," are withdrawn ; but the expanded phase is again 

 resumed on the morrow. The white background contracted 



