THE WONDER OF LIFE 545 



milky and translucent; the chromatophores contract 

 greatly, and there is an unexplained turbidity in the cara- 

 pace. When it is put into a glass vessel and that placed 

 on a mirror, it becomes transparent, the maximum con- 

 traction of chromatophores occurs. It is easy to imagine 

 conditions where this miUdness or this transparency would 

 be very useful. On the other hand, we read that an 

 individual forced to jump loses its transparency, which 

 does not soimd so adaptive. 



Professors Gamble and Keeble have demonstrated a 

 remarkable plasticity in the coloration of the -^Esop-prawn 

 {Hippolyte varians), which may be red, yellow, blue, orange, 

 olive, violet, brown, green, and other colours. It is born 

 without a bias, and it takes on the hue of its environment, 

 both when young and adult. If it is put in an aquarium 

 the sides and floor of which are lined with coloured paper, 

 it takes on the colour ; and it wiU change from one colour 

 to another. It seems to have more plasticity in its color- 

 ation than it can possibly need, but it can make itself 

 invisible among the bright colours of seaweed. 



In the j^sop-prawn the colour changes periodically 

 with the nervous state of the animal, according as it is 

 sleepy or wakeful. We venture to quote Professor Gamble's 

 fine description {The Animal World, p. 140) : 



' The wakeful hours of Hippoljrfce are hours of expansion 

 The red and yellow pigments flow out in myriads of stars 

 or pigment-cells ; and according to the nature of the back- 

 ground, so is the mixture of the pigments compounded to 

 form a close reproduction both of its colour and its pattern : 

 brown on brown weed, green on Ulva or sea-grass, red on 

 the red Algae, speckled on the filmy ones. A sweep of a 

 shrimp net detaches a battalion of these sleeping prawns, 



