114 MICROSCOPICAL STUDIES. 



retain the power of changing their colour in accordance with that of 

 their surroundings. Thus a pale olive brown H. varians taken from 

 amid similarly coloured sea-weed became of a vivid green within an 

 hour when placed with Enteromorpha, and the same specimen changed 

 to a pinkish red within three hours when placed amid Delesscria. 



Again, red coloured specimens of the same species from amongst 

 tufts of red weed changed to green during a single night when placed 

 with Enteromorpha or with Cladophora, and back again to red within 

 four hours when placed once more amid red weed. 



These two instances are representative of a large number of similar 

 experiments, all having parallel results and demonstrating that the 

 adult Hippolyte varians has great and rapid colour adaptability. 



It is a remarkable fact that this change of hue takes place as 

 rapidly in the dark as in the light ; thus specimens placed in a dark 

 cupboard in company with weed of a different colour, will be found to 

 have assumed the colour of the weed in the course of a few hours. In 

 the same way individuals left over night exhibit their appropriate 

 colour change when examined before daybreak on the following morning. 

 As to Hippolyte fascigera, I find that this species possesses by no 

 means equal colour adaptability ; this power is much less developed. 

 Thus it takes considerably longer time to adapt its hue to new colour 

 surroundings, and the colour is but an imperfect approximation to the 

 true one. For example, the majority of this species, which are found 

 living among the mottled pink coarse Corallina of our pools, and have 

 absolutely similar colouring, take fully twelve to fifteen hours to 

 approximate partially to the white tint of sun-bleached tufts of weed. 



Again, a lengthened sojourn of upwards of a week amid tufts of 

 Enteromorpha does not effect any well marked assumption of green, 

 though the pink is decidedly paler and a tinge of green can be made 

 out without difficulty. 



Such marked divergence in this power of colour adaptability between 

 two species so closely akin* is extremely remarkable, and I had to 

 make a very careful examination of the natural habitats and habits of 

 the species in question ere I was able to solve the problem. 



I find that H. varians is infinitely more numerous and more widely 

 spread than is H. fascigera, being found in almost every pool, and 



* Indeed, so closely akin that a cursory examination is apt to lead to the belief 

 that H. fascigera is a mere variety of H. varians. Many points of permanent 

 divergence are, however, present ; the most marked is that in II. fascigera the only 

 spines on the upper edge of the rostrum are three placed at the posterior end and 

 really upon the carapace, while a single sharp tooth is set close to the tip on 

 the straight under edge. In II. varians, the rostrum above has one tooth set near 

 the base and another towards the tip, while below there is a well-marked two- 

 toothed keel. 



