But the cells need air as well as food; and, to carry this, there are little basket-cells--the _red corpuscles_.

Take a drop of blood and put it under a microscope, and you will see what they look like.

The field will be simply crowded with tiny, rounded lozenges--the red cells of the blood, which give it its well-known color.
[Illustration: BLOOD CORPUSCLES (Greatly magnified) _A_, red blood; _B_, white blood.] The White Corpuscles or Scavengers of the Blood.

As the blood-tubes are not only supply-pipes but sewers and drainage canals as well, it is a good thing to have some kind of tiny animals living and moving about in them, which can act as scavengers and eat up some of the waste and scraps; and hence your microscope will show you another kind of little blood corpuscle, known, from the fact that it is not colored, as the _white corpuscle_.

These corpuscles are little cells of the body, which in shape and behavior are almost exactly like an _ameba_--a tiny "bug," seen only under the microscope, that lives in ditch-water.