628 P. W. GAMBLE AND P. W. KBEBLE. 



and the recovery from it postponed. In the previous sections 

 we proved the sensitiveness of Hippolyte varians to 

 changes of light-intensity. By exposing prawns in muslined 

 or porcelain jars, or in white dishes, to bright light in the 

 open or to incandescent gas-light, a change of colour to green 

 was shown to be induced. We can now show that by con- 

 tinuing the experiments on freshly caught specimens till 

 evening, the green colour deepens into the blue nocturne 

 phase. In fact, the readiest mode of inducing the nocturnal 

 condition is to place some prawns in a white jar, to cover the 

 jars with muslin, and to add only a scrap or two of weed 

 to serve as food. Placed under a current of water the 

 specimens assume the nocturnal hue fully an hour or more 

 before others in clear glass vessels. The following morning, 

 at an hour when other prawns freely exposed to diffuse 

 light have fully recovered, some of those in muslined jars 

 are still nocturnes, and these generally exhibit a preference 

 for clinging to the under surface of the muslin close to the 

 light. Similar experiments made in December, 1898, with 

 flasks enclosed in a double fold of white muslin, with and 

 without green weeds, show that a green colour becomes 

 habitual with originally olive, blackish, and brown specimens, 

 and that this colour readily passes into the nocturnal tint 

 during twilight. It would seem, indeed, that in captivity 

 specimens live better in winter than in summer, and acquire 

 and retain their nocturnal colour more readily. 



Scattered white light, then, predisposes the prawns to 

 change in the nocturnal direction. In this light, which is 

 markedly different from that which the animals experience 

 whilst hanging or lying motionless on their weeds, the colour- 

 patterns, which their habitat has in some way or other im- 

 pressed on them, are lost. Nothing could more clearly show 

 that the maintenance of the colour-patterns in the normal 

 prawns is the work of the nervous system ; though why the 

 nervous system fails to maintain the sympathetic coloration 

 under the peculiar light-conditions just indicated is beyond 

 us to explain. 



