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Fig. 1.—Microscopic views of a shelled amœba.
A medium-sized amœba is over a million million times smaller than the smallest mammal.
Shells of two diverse races of Difflugia corona, showing characteristics that become hereditarily altered as generations pass. The individual shown at a and b (a side view and a view looking at the mouth) bears about seven long spines and has sixteen teeth around the mouth; the individual shown at c and d has four small spines and fourteen teeth.
