Protozoa (Gr. , first and , animal), a subdivision of invertebrate animals, proposed by Siebold, since adopted by Leuck-art and Vogt, and now generally admitted by naturalists. As they include the lowest and in most cases the most microscopic of animals, the limits of this division are not well defined; they comprise many of the so-called animalcules, as well as the large sponges. They are composed of a nearly structureless, jelly-like substance, called protoplasm or sarcode, without distinct segments, internal cavity, or nervous system, and with no or a very rudimentary digestive apparatus. (See Protoplasm.) Dr. Engelmann has observed in arcella, a minute amoeba-like protozoan, a periodical development of gas in the granular protoplasm, unconnected with the contractile vacuoles or the nuclei. He thinks this is a voluntary act, and that the bubbles are used in the manner of a float or air bladder. Its chemical composition and the mechanism of its production and absorption were not determined. The usually accepted division is into the classes of gregari-nidoe, rhizopoda (like amoebae, foraminifers, and sponges), and infusoria, the highest, with a mouth and digestive apparatus, like the bell animalcules and paramoecium.
As these represent the first step in animal organization, so do the protophytes the first in vegetable life; the former were called oözoa by Carus, from their resemblance to the ova or germs of higher animals; the latter, as far as known, were microscopic seaweeds, without the radiate structure characteristic of plants, and are found in the lower Silurian strata. (See Animalcules, Bathybius, Coccoliths, Forami-nifera, Globigerina, and Gregarina.) - See Prof. Packard on the "Development of Protozoa," in the "American Naturalist," December, 1874, to February, 1875.