protozoa

Century Dictionary and Cyclopedia

1. Primordial or first-formed animals, or cell-animals; protozoans: a subkingdom of Animalia or prime division of animals, contrasted with Metazoa, or all other animals collectively. The Protozoa are animal organisms consisting of a single cell or of several cells not differentiated into tissues. This is the essential distinction between protozoan and metazoan animals, though no hard and fast line can be drawn around Protozoa to distinguish them on the one hand from Protophyta, and on the, other from Metazoa. The name Protozoa was first used by Goldfuss (1809) to include microscopic animals and also the polyps and medusæ. Siebold and Stannius first used ii in its modern signification as comprising and limited to the infusorians and rhizopods. Owen (1859) used the term Protozoa for a kingdom including diatoms, etc., and therefore synonymous with Pretista. The sponges, in the view (as held by W. Seville Kent, for example) that they consist essentially of an aggregate of choano-flagellate infusorians, are often brought under Protozoa, though they have not only an ectoderm and an endoderm, but also a mesoderm, and are therefore, tissue-animals as distinguished from cell-animals. Excluding sponges, Protozoa may be characterized as animals composed of a simple nearly structureless jelly-like substance called sarcode, a kind of protoplasm, devoid of permanent distinction or separation of parts resulting from tissue-formation or histogenesis (though they may have very evident organs as parts of a single cell), without a permanent definitive body-cavity or any trace of a nervous system, no permanent differentiated alimentary system except in a most rudimentary slate, and no multicellular membranes or tissues. Nevertheless, there is really a wide range of variation orgradation of structure in these, seemingly structureless animalcules. Some of the lowest forms are mere microscopic specks of homogeneous sarcode, of any or no definite shape. Such are moners, or representatives of a division Monera or Proteomyxa; but it is not certain that all such objects are either individuals or species in a usual sense of these words. Among the lowest protozoans of which species and genera can be definitely predicated are the amœbiform organisms, which have a nucleus, and locomotory organs in the form of pseudopods, temporarily protruded from any part of the body, and which ingest and egest foreign substance from any part of the body. Vast numbers of protozoans are of this grade of complexity, and with the simpler forms constitute a class, Rhizopoda, including the normal amæboids and the foraminifers and radiolarians. For, though both these latter may have very complicated shells, tests, or skeletons, their sarcodous substance remains of a low and simple type. It is an advance in organization when a protozoan becomes corticate — that is, assumes a form in which an outer harder ectoplasm and an inner softer endoplasm are distinguishable — since this confines the sarcodous mass and gives it definite shape or form. This advance in organization is often marked by the appearance of a nucleolus or endoplastule, besides the nucleus or endoplast which most protozoans possess, by the presence of definite and permanent locomotory organs in the form of cilia or flagella, and finally by the fixation of a specialized oral oringestive area or mouth, in place of the one or several temporary vacuoles which serve as stomachs in lower forms. Protozoans of this higher grade occur under various forms. The class Gregarinida represents parasitic forms, one- or two-celled, essentially like the ova of Metazoa. The class Infusoria comprehends an enormous number of minute, nearly always microscopic, animalcules, found in infusions, inhabiting both fresh and salt water, sometimes parasitic, but mostly leading an independent fixed or free life. There are many groups of these, as the ciliate, flagellate, choanofiagellate, and suctorial infusorians, among them the most complex organisms which are commonly included under Protozoa, as the Noctiluca, for example. With or without some of the lowest disputed forms, and with or without the sponges, Protozoa have been very variously subdivided, almost every author having his own arrangement. A so-called moner, an amœba, a foraminifer, a radiolarian, a gregarine, and an infusorian respectively exemplify as many leading types of Protozoa. One division is into Automata and Stomatoda, according to the absence or presence of a mouth. Another is into Monera and Endoplastica, according to the absence or presence of a nucleus, the latter being again distinguished as Myxopoda and Mastigopoda, according to whether the locomotory organs are temporary pseudopods or permanent cilia or flagella. A third is into Gymnomyxa and Corticata, according to the absence or presence of a distinguishable ectoplasm.

Wiktionary

1. n. Plural form of protozoan.
2. n. Plural form of protozoon.

GNU Webster's 1913

1. n. (Zoöl.) The lowest of the grand divisions of the animal kingdom.

WordNet 3.0

1. n. in some classifications considered a superphylum or a subkingdom; comprises flagellates; ciliates; sporozoans; amoebas; foraminifers.