Protozoa

Protozoa (Gr. prōton, 'first,' and zōon, 'animal'), simple unicellular animals, contrasted with the multicellular Metazoa. Except in a few cases, each Protozoan is a single cell, a unit-mass of living matter physiologically complete in itself. Being such a unit involves being without organs and without sexual reproduction. Yet a Protozoan may have parts, and two individuals may unite in mutual fertilisation. A Protozoan is to any higher animal, from sponge onwards, as an egg-cell is to the body into which it develops. But the exceptional cases to which we referred are most important—they are loose colonies or aggregates of Protozoa. Formed by the incomplete separation of dividing units, they bridge the gulf between single-celled and many-celled animals. Simplest of Protozoa are such forms as Protonyxia, whose life is a succession of changeful phases, amoeboid, encysted, flagellate. The others may be classified according to the predominance of one or other of these phases. The Rhizopoda, predominantly amoeboid, include Amœba and others like it, Foraminifera, Heliozoa, and Radiolaria. The Gregarines are predominantly sluggish and encysted. The Infusorians are usually active, ciliated, or flagellate. These classes of Protozoa are discussed separately.