HIPPOLYTE VAUIANS. 641 



Further^ as black coffee served out to a number of meu may 

 here aud tliere succeed in effecting wakefulness, so we find 

 that in a batch of prawns subject at evening to high light- 

 intensity some individuals maintain their diurnal colours and 

 habits, whereas others are proof against the excitant. Indeed, 

 in devising the experiment of coustant illumination we anti- 

 cipated that the prawns would, by reason of their periodicity, 

 resist the stimulus of light, and were delighted to find our 

 expectations realised. 



It may be urged that in the colours of deep-sea Crustacea 

 we can observe the effects of long-continued darkness, and 

 that such forms exhibit no permanent nocturnes. Now, in 

 the first place, we are not prepared to assert that continued 

 darkness does induce permanent nocturnes. We know that 

 other organisms exhibiting periodicity pass, after a certain 

 exposure to constant-light conditions, into a state of rigor. 

 In this state of rigor induced by constant darkness, the 

 organism may assume a condition more like that which it 

 normally displays during the day. So it may possibly be with 

 Hippolyte. The colour-rigor, if such occur, may possibly 

 be more like the diurnal than the nocturnal colour-phase, 

 though the observations of Professors Brooks and Herrick, 

 already cited, onPalgemonetes varians tend to show that, 

 in the case of that animal, a condition akin to that of our 

 "nocturnes" is effected by prolonged darkness. In the 

 second place, although the brilliancy aud depth of tone 

 of the red coloration of the arctic and Norwegian deep- 

 sea species of tliis and allied genera, adds point to the 

 possibility which we have just indicated, yet we suggest that 

 before the colours of deep-sea forms can be appealed to, 

 these colours must be observed with more precautions than 

 have yet been taken. We have seen how the colour of a 

 full nocturne may give place almost instantaneously to the 

 diurnal condition; hence, before any statement can be made 

 as to their natural colour, the deep-sea Crustacea must be 

 protected from light and other disturbing influences on 

 their way to the surface. Even " shock,^' as we shall show 



