The Colour-Physiology 0/ Hippolyte variaus. 467 



intensity to which the prawns are exposed, or by subjecting them to 

 light reflected from white, and especially porcelain surfaces. Third, 

 the sympathetic colour-change brought about by change in the colour 

 of the surroundings. 



The first of these is habitual or periodic, and may be quickly pro- 

 duced, towards evening, by a profound alteration of the light-intensity. 

 Even the natural recovery from the nocturnal to the diurnal colour 

 takes place rapidly with the dawn. The second change, as inexplicable 

 in teleological terms as the first, is also rapid, often very rapid. 



The third change is extremely slow. The prawn, in the acquire- 

 ment of its adult colour, is guided and guided solely, so far as 

 external circumstances are concerned, by light-intensity. In response 

 to the conditions of light-intensity which prevail in its habitat, the 

 prawn metabolises and distributes its pigments. But its pigmentary 

 forces do not admit of ready mobilisation for purposes of defence ; or 

 at least they do not quickly obey a command to move. Hippolyte 

 by its immobility has gradually grown into its surroundings and 

 though, as for example, at night, its pigments may be readily aggre- 

 gated, and a special nocturnal colour produced, yet this mobilising 

 power is not utilised at all, or but very slowly, to redistribute the 

 pigments, when the colour of the habitat is changed. 



VIII. The Causes of Change of Colour. 



(a) Hippolyte grows into harmony with its surroundings. So 

 developed it hangs on to wave-swept weeds. Should it be dislodged 

 its hope of concealment lies rather in a rapid choice of a weed of its 

 own colour, than in a slow sympathetic colour-change on its own part, 

 for if we may trust our experimental results, a week would elapse 

 before the change could be complete. Monochromatic light (obtained by 

 the use of Landolt's colour-filters), is singularly inefficacious in pro- 

 ducing any sympathetic colour-change. Red, yellow, green, and blue 

 light act in this respect like darkness. Under natural conditions Ave 

 conclude, therefore, that the ultimate colour-change is effected by a 

 reaction to light-intensity. 



(b) After much trouble and many experiments, we find that there is 

 no evidence that rays of light, by virtue of their specific wave-lengths, 

 play any part in changing the colour of the prawns. On the other 

 hand we find that in the diurnal phase, a low light-intensity favours 

 expansion of the red pigment and so brown effects, while increased 

 light-intensity produces a green tint. The appreciation of light-inten- 

 sity appears to be very acute and to be the chief agent in producing 

 colour-change. 



(c) By ablation of the eyes, electrical stimuli, and heat, colour- 

 changes may be induced. These agents have been employed in tracing 



