180 THE SEAS 



in crustaceans, in the squid and cuttlefish family, and in 

 fish. Amongst the first of these the most striking example, 

 which has been thoroughly investigated by Professors 

 Gamble and Keeble, is that of the little iEsop prawns 

 (Hippolyte). These little creatures, never more than an 

 inch in length, are very common on seaweed in rock pools 

 and on rocky coasts but are very difficult to discover 

 owing to the exactness with which they tone with the 

 background. 



Beginning with the birth of the prawn ; when the young 

 hatch out they are colourless and almost transparent and 

 drift about in the sea for some time, being finally carried 

 inshore where, for the sake of the food and security 

 afforded them, they cling to the first piece of seaweed they 

 meet. They now begin to develop colour, a process which 

 takes place quickly and always results in the animal 

 assuming the exact shade of colour of the weed which it 

 has chosen for its home. If this is a red weed from deeper 

 water, the prawn becomes red, if a brown weed from the 

 intertidal zone, it becomes brown, or if a green weed in 

 the rock pools near high-water mark, it becomes green 

 (Plate 66). The process begins gradually but at the end 

 of a week the match in colour is usually perfect and the 

 prawn almost impossible to detect. 



If the iEsop prawn is driven from its home by unfavour- 

 able conditions, such as strong tides or currents, it searches 

 for a new home of the same colour, but should this be 

 impossible (as it may easily be made in the laboratory 

 by placing collected prawns on weeds of different colour 

 from themselves) then the prawn changes its colour to that 

 of the new home and at the end of a week will have 

 developed a new colour scheme just as exact an imitation 

 of its surroundings as was the first. Quite independent 

 of these enforced changes of colour due to change of sur- 



