346 ADAPTATIONS. 



surface of the reefs, as the Tridacna, and those great uni- 

 valves, such as the Turbos ; or, in short, those which 

 always remain motionless, like the Limpets, which are 

 attached, by forming a vacuum on the smooth surface of 

 rocks : all these species of shells are of the colour of the 

 bottoms, or floors, which they respectively inhabit, in 

 order, no doubt, that they shall be less perceived by their 

 enemies. 



