PBOTOTHERIA. 473 PROTEACTOE. brain. The skeleton exhibits many archaic fea- tures, one of the most striking of which is the presence of only the capitular head to the ribs. The shoulder girdle has other reptilian features, and a bone (the interclaviele) peculiar to the group. The digestive and circulatory systems differ .little from the noiTnal mammalian type, and the great distinction between the Prototheria and other mammals lies in the reproductive sys- tem, and the fact that their eggs, instead of being minute and with little or no food-yolk, are large, contain much yolk, and therefoie develop, so far as their early stages are concerned, after the meroblastic manner of a reptile's egg. The Pro- totheria have a temporary ventral 'mammary' pouch in which the young are hatched, or to whifh they are transferred after hatching, and into which open the ducts of the mammary glands. This pouch is formed by a deep fold of the skin periodically develojied in prepara- tion for the young, and contains no teats, but nutrition is supplied by modified sweat- glands. Its homologies are fully discussed by Beddard {Mammalia. 1902), who refers to the work and writings of many investigators. See DlCKIIII.L: ECHIDXA; ilAlIilALIA'? PROTOZOA (Xeo-Lat. nom. pi., from Gk. irpwTos, /jio(os, first -I- fuov, zoon, animal) . The subkingdom or phylum of one-celled animals, represented by the amoeba (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 fonn. Protozoa consist of a single cell, and. with the exception of the moners (q.v.), they possess one or more nuclei, but no other organs 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, ilotion is not only brought about by the general contraction of the body, but by means of "pseudopodia' (see AmcebaI. 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 fresh-water 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 Reproductiox : Sex.) They may be naked, or secrete a calcareous shell, consisting of one. two, three, or many chambers: in the latter ease (the Foraminifera ) , the shell is remarkably complex, considering the great simplicity of the animal itself. Ap- parently the same 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 (Amaba terricoln) . 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- hoek (q.v.), in I7G-1 first detected and described certain forms living in infusions, and about the same date Wrisberg {Observationes de Ani- malculis Infusoriis, Gottingen, 1705) called them 'infusorial animalcules.' The name Proto- zoa was given in 1S4.5 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 hf ilatthews in the Lower Cambrian rocks of Saint John, X. B. It is to be observed that if xe 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 Eadiolaria date from the Cambrian. See Fobasiinifeb.; R.^diolabia. Classification. The phylum Protozoa is di- vided into ^ve classes: (1) Rhizopoda : (2) ilycctozoa; (3) Flagellata ( ilastigophora) ; (4) aporozoa ; and (5) Infusoria. Of these the Sporozoa are all parasitic, while the malarial germ is a protozoan of doubtful position, its young being provided with flagella. The latest work is Lankester, A Treatise in. Zoolofii/. part i. (London, 1903), which contains a bibliogiajjhy. PROTRACTOR OIL. protractor, from Lat. protrahere, to draw forward, from pro, before, for -f- trahere, to drag. draw). An instrument used for measuring and laying down angles on paper. In its simplest form it consists merely of a semicircular scale of metal or transparent ma- terial. The three-arm protractor used in ma- rine surveying is used to solve mechanically the "three-point problem.' The middle arm is fixed with its reading edge at the zero of the scale; the other arms, pivoting at the centre of the in- strument, are arranged to measure angles on each side of the middle arm and all carry ver- niers. The method of using the protractor is as follows: Three objects (whose positions are marked on the chart or map) are selected. The angle between the right and centre objects and that between the centre and left are measured with sextants or similar instruments. The angles so obtained are transferred to the pro- tractor, which is then laid upon the chart, and, with the edge of the centre arm always kept on the marked position of the centre object, the instrument is slipped along until the side arms fall upon the positions of the other two. The centre of the protractor then indicates the posi- tion occupied by the observer when the angles were taken, and crossed wires, a hole in a glass- ended tube, or a needle point serves to fix this position, which, if the angles are simultaneously observed, can be accurately ascertained even when the surveying boat is moving at a higher
