Crustaceans
a class of articulated animals, agreeing with insects, arachnida, and myriopoda, in having articulated limbs, but differing from them in important respects, and particularly from all of them in the adaptation of the organs of respiration to an aquatic life, even those of them which live on land being generally inhabitants of damp places, and breathing by a kind of gills.
Some of the lowest and minute aquatic crustaceans, indeed, are not provided with gills; but with the aeration of the blood is supposed to take place through the surface of the body. The crustaceans derive their name from the hard armor which in most of them covers their whole body.