HIPPOLTTE VARIANS. 633 



pigments is giving place to the flow, tlie animal may be seen 

 green, or, if the flow be at the fall, reddish brown or brown. 

 The prawns in the dark are shown, in the next section, to be 

 continually passing to and from the nocturnal condition, and 

 their colour at any moment depends on the point which the 

 colour pendulum has reached in its periodic swing. If there 

 is any colour phase more stable than another, it is that 

 which characterised the animal during its last light experi- 

 ence, and hence it is to that colour-form that the animal 

 most frequently reverts after a nocturnal bout. 



Whether the nocturnal condition in any way favours 

 adaptive coloration, whether it be but a defect of the quality 

 of a nervous system highly strung to colour attunement, a 

 phenomenon of fatigue, or whether it be but a reminiscence 

 of a pelagic habit either of the species or of some period in 

 its own life history, we have no means of deciding. We have 

 contented ourselves with describing one of the most beautiful 

 and striking sights imaginable, a medley of colours swiftly 

 passing into one harmonious hue. 



Conclusions on the Nocturnal Phase. — Contrary to 

 Pouchet, we find that night induces a very distinct phase in 

 the cycle of colour-changes of which Hippolyte varians 

 is the subject. The same striking contrast between diurnal 

 and nocturnal coloration is exhibited by species of Mysidge 

 and by Pandalus. Pouchet himself, while denying, as we 

 have seen, that darkness has any effect in altering the 

 " fonction chromatique " in Crustacea, cites a couple of 

 experiments, one with two young lobsters, one red and one 

 blue, and the other with some dark, freshly caught shrimps, 

 in which darkness produced in the red lobster a blue, in the 

 shrimps a white coloration (1876, p. 152). 



The nocturnal phase is distinguished by the disappearance, 

 or rather retraction, of all save the peculiar blue pigment, and 

 is associated with great transparency of the tissues. In some 

 cases the blue colour of the network is suppressed, and an 

 almost colourless or greyish appearance results. The phase 

 commences as darkness sets in, attains its full development. 



