The Rhizopoda may be defined as Protozoa which are destitute of a mouth, are simple or compound, and possess the power of emitting "pseudo-podia" They are mostly small, but some of the composite forms, such as the sponges, may attain a very considerable size. Structurally, a typical Rhizopod - as an Amoeba - is composed of almost structureless sarcode, without any organs appropriated to the function of digestion, and possessing the power of throwing out processes of its substance so as to constitute adventitious limbs. These are termed "pseudopodia," or false feet, and are usually protrusible at will from different parts of the body, into the substance of which they again melt when they are retracted. They are merely filaments of sarcode, sometimes very delicate and of considerable length, at other times more like finger-shaped processes; and they are identical with the little processes which can be thrown out by the white corpuscles of the blood and by pus-cells. Indeed, it has been remarked by Huxley that an Amoeba is structurally "a mere colourless blood-corpuscle, leading an independent life."

The class Rhizopoda is divided into five orders - viz., the Monera, the Amoebea, the Foraminifera, the Radio/aria, and the Spongida, of which the last is occasionally considered as a separate class, or is removed entirely from the Protozoa.