PROTOZOA (Neo-Lat. nom. pl.. from Gk.
rp&ros, protos, first soon, animal). The subkingdom or phylum of one-celled animal-, represented by the anweba (q.v.), infusoria. and the like. They are. as a rule, of microscopic size, and are like particles of protoplasm (q.v.), having a gliding motion and constantly chang ing their form. Protozoa consist of a single cell, and, with the exception of the mouers (q.v.), they possess one or more nuclei, but no other or true tissues. As the entire body is composed of protoplasm. the simplest proto zoan is contractile, absorbs and digests food, is metabolic. automatic, and reproduces by self division. 'Motion is not only brought about by the general contraction of the bOdy, but by means of 'pseudopodia' I see AmcEnx). cilia. and a specialized cilium called 'flagellum.' while in the most specialized infusoria, such as Vorti cella and stentor, little muscular fibrilhe have been detected. Besides the nucleus and food vacuoles, there are other cell-organs called 'con tractile vacuoles.' which occur in freshwater forms and only rarely in marine species. They apparently perform an excretory function, and may be respiratory. since they are supposed to eliminate carbon dioxide. All the vital func tions appear to be under the control of the nucleus. Protozoa reproduce by self-division Or budding, or they conjugate. multiplying by spores or germs. (See REPRODUCTION : Sex.) They may he naked. or secrete a calcareous shell, consisting of one, two. three, or many chambers: in the latter case I the Foraminifera the shell is remarkably complex, considering the great simplicity of the animal itself. Ap parently the mechanical laws guide the mode of shell-formation. the chambered shells being irregular, or straight. or twisted. or coiled in a single plane. like the chambered nautilus.
While a very few forms are terrestrial (..1nrba terricola), the vast majority are marine and fresh-water forms, the shelled forms being ma rine. The fresh-water forms abound most in still or stagnant water, and may become encysted when the water dries up. or when food is lack ing or cold approaches. Thus protected by a thin resistant outer covering. the monads and infusorians in general may dry up and be blown about by the winds, remaining suspended in the air for a long period. In this way the species have become more or less cosmopolitan. A large proportion of American forms are of the same species as those of Europe.
The discoverer of the microscope, Leeuwen hock (q.v.), in 1764 first detected and described certain forms living in infusions, and about the same date Wrisberg (Obscrrationcs de dni nnaleanix In! usoriis, Gottingen, 1765) called them infusorial animalcules.' The name Proto zoa was given in 1S45 to the subkingdom by Sie bold, who discovered that they were unicellular, disproving Ehrenberg's claim that they pos sessed a digestive canal, nervous system, muscles, excretory and sexual organs. The earliest traces of shelled Protozoa are those of Globigerina and Orbulina detected 14- :Matthews in the Lower Cambrian rocks of Saint John, N. B.
It is to be observed that if we detect the re mains of shelled Protozoa in the Cambrian for mation. it is safe to suppose that the seas and fresh waters of that early period harbored nu merous soft-bodied or shell-less forms. It is also supposed that the present types of Protozoa are very early forms which have persisted from Cambrian times, the type having undergone but little specialization. The Badiolaria date from the Cambrian. See FOBAMINIFEIIA RADIOLABIA.