HOW ANIMALS ADAPT THEMSELVES 23 



colour, fairly bright. When seen from below, 

 with the sunlight streaming through them, they 

 no doubt appear much brighter than when seen, 

 as we see them, from above, with the sunlight 

 falling on them. Now protection from foes below 

 is what the yellow periwinkle needs most : for 

 fishes are quite ready to swallow it whole, and 

 are not in any way deterred by the thickness of 

 the shell, which is (by-the-way) in a measure a 

 protection against birds when the tide is out ; 

 fishes habitually swallow shell-fish whole, al- 

 though the inmate only is digested. The bright 

 yellow, then, that seems to us so conspicuous, is 

 probably a good means of hiding for the peri- 

 winkle when under water. Its common varia- 

 tions in colour, too, are probably protective in 

 their use: some are a dull purplish brown, some 

 drab. These are good colours in which to lie 

 hidden, respectively, under darker tracts of sea- 

 weed, or upon the rock itself. This little shell is 

 so abundant on rocky coasts that on some beaches 

 the dead shells are as numerous as pebbles. No 

 wonder, with all these adaptations for protection! 

 Another instance of adaptation to circumstances 

 is described in the sea-urchin shown on p. 125. 

 This is one among many instances where animals 

 that live on sand or mud acquire a flattened 

 shape, so that their weight is distributed, and the 

 danger lessened, of their sinking in a quick-sand. 

 The flat-fish, such as soles and flounders, are a 

 familiar example ; and the same principle is 

 illustrated by the flattened forms of many of 

 the bivalve shell -fish, whose flat shell, when 

 closed, can lie safely on the loosest sand. 

 Equally is their form adapted for their circum- 

 stances, when, in their slow way, they begin to 



