I 7 4 ANIMAL LIFE 



Their youth, like that of higher animals, is shaped 

 by early impressions, which come with a force and are 

 responded to with an alacrity that diminish in later 

 life. 



Across their sensitive, transparent skin a shadow 

 falls. Scattered over the body, some pigment, in- 

 significant save under a microscope, is present, and 

 in the shadow's path this pigment undergoes a change. 

 By a rule of behaviour that governs the expansion 

 of pigment into interwoven skin-channels, or its 

 contraction into pores, the shaded pigment will 

 expand and darken that area of the body. Further, 

 as exercise favours growth, so does this expansion 

 of pigment seem to induce increase in its amount. 

 Between the existing dots of colour new ones arise, 

 the bar thickens and in a few days reproduces the 

 shadow in tone. 



Thus we can trace the influence of light and shade 

 in preventing or favouring the development and 

 expansion of skin-pigment in Hippolyte, and in 

 thereby producing that sympathy between the prawn 

 and its surroundings that afford it such wonderful 

 concealment. 



The development of the colouring, as well as of 

 the light and shadows, at this critical phase is less 

 well understood. We know that the red and yellow 

 pigments are older than the cryptic colouring they 

 produce; that their value as nourishing, if not also 

 as respiratory, mechanisms explains their presence 

 and antedates this merely decorative effect. But by 



