692 F. W. GAMBLE AND F. W. KEEELE. 



prietes actiniques du milieu ambiant^' makes it doubtful 

 liow far Pouchet attributed colour-change in the prawn to the 

 change in tint of the bottom^ in other words to the varying 

 quality of the light, or to the light intensity, or to both 

 these possible causes. Other passages in the paper show 

 that Pouchet attributed to the varying quality of the light 

 reflected from the differently coloured surfaces on which 

 the prawns happened to be resting, an important and perhaps 

 the chief share in the resulting colour-change. But there 

 is still another of Pouchet's conclusions, which tends to 

 show that he regarded the varying quantity of the light as at 

 least a subsidiary factor which, acting through the eye and 

 nervous system on the pigment spots, may modify the colour. 

 He says (p. 151), " L'obscurite de la nuit n'a aucune influence 

 sur les changements que presentent les animaux, tandis que 

 l'obscurite artificielle provoque au contraire des modifications 

 qui pour n'etre pas constantes ni toujours bien definies n'en 

 sont pas moins sensibles." This conclusion Pouchet drew 

 from his observations on fishes (turbot, goby, dragonet) as 

 well as from those on Crustacea, and it is on this point more 

 than on any other that we find ourselves opposed to him. 

 It would, indeed, be singular if an animal were sensitive to 

 change in light intensity, and yet remained the same colour 

 by night as by day. In the sections on nocturnal colour and 

 on periodicity we show not only that Hippolyte varians 

 undergoes a rhythmical sequence of colour-change composed 

 of nocturnal and diurnal phases, but that this is a habit which 

 persists though the light or the darkness is maintained con- 

 stant for days together. 



The effects of varying light intensity on Crustacea have 

 attracted but little attention since Pouchet's memoir in 1876, 

 Matzdorff (1882) has experimented with Idothea and con- 

 firmed many of Pouchet's conclusions. Jourdain (1878), 

 working on Nik a edulis (a prawn belonging to a family 

 distinct from the Hippolytidee), obtained some interesting 

 results. This form, he found, passed from a translucent 

 nearly colourless condition in sunlight or diffuse light to a 



