Prō'to-zō'an, n.

n. [Gr. πρῶτος, first, and ζῷον, an animal.] (Zool.) An animal of the lowest of the five grand divisions, distinguished by its simplicity of structure, neither of the four systems of structure characterizing severally the other grand divisions being strongly marked. See ANIMAL KINGDOM.

☞ The protozoans, generally, have no mouth, or only such as may be formed by a depression of the surface at the time when a particle of food is to be received and digested. They include the rhizopods, sponges, and many of the so-called animalcules; the sponges and most rhizopods being compound groups of protozoans, each produced by growth and a budding process, and some of them extending out a fiber-like process from the surface, whence the name rhizopod, used for one group. The principal subdivisions are the following: 1. Actinozo-oids, or radiate protozoans, including the sponges and polycistines, the mineral secretions of which are almost always silicious, and the structure of whose shells is radiated. 2. Malocozo-oids, or mollusk-like protozoans, as the rhizopods, the shells of which, when any exist, are calcareous and spiral or alternate in the arrangement of the cells. 3. Entomozo-oids, or articulate-like protozoans. The animalcules that are not plants or leaves are supposed to belong to one of these three subdivisions. Dana.