l6 THE STORY OF ANIMAL LIFE 



uous; brown being, in fact, the contrast-colour 

 which shows blue in its greatest brilliancy. Yet, 

 when perched upon the tangle, the creature is 

 almost invisible, and might easily be mistaken for 

 a natural irregularity of the surface of the sea- 

 weed. While the brown is the colour of the sea- 

 weed itself, the brilliant blue is indeed the exact 

 colour of the spring sky at that season, every- 

 where reflected from the sea-water and from the 

 wet surface of the seaweed. By matching that 

 brilliant colour the animal therefore is rendered 

 invisible. This little creature is the young of 

 the Semi-transparent Limpet, Patella pelhtcida. 

 This, at least, was the old-fashioned name for 

 it, though it has received others. Its young 

 and its adult form are so different in the ap- 

 pearance of the shell, that they have been de- 

 scribed under different names. English readers 

 who search for it in the spring will learn by ex- 

 perience that bright colouring may help to make 

 a creature invisible. But this is not all that is to 

 be said about the protective colouring of this 

 little shell-fish. There are many creatures whose 

 young live at the surface of the sea, and after- 

 wards migrate to deeper water as they attain 

 adult age. In early life they are transparent, 

 because thus they best escape notice in the clear 

 water of the surface, especially when seen from 

 below, by the many enemies on the watch to de- 

 vour them. But in their later life they become 

 opaque, because thus they best escape notice from 

 enemies watching from above, as they crawl along 

 the bottom of the sea. Now this is the case with 

 the little Patella. For this also migrates to the 

 bottom in this instance a comparatively short 

 journey when it is ready for adult life. Both 



