36 PROTRACTOR internal nucleus and external protoplasm by the plasson bodies of moners. Edouard van Beneden especially calls for this distinction in his Recherches sur revolution des gregarines ; and Haeckel has adduced new facts in favor of it in his Monographic der Ealkschwdmme. For the theory of " primordial generation," the spontaneous generation of the first vitality on earth, the distinction is of special importance, aa the first organisms thus produced could have been only structureless specks of plasson, like the bathybius and other moners. The great theoretical difficulties formerly in the way of the theory of primordial or spontaneous gener- ation have been removed by the discovery of the moners and the establishment of the plas- tid theory. As the protoplasm of the bathy- bius is not yet as much as individualized, while in the case of other moners there are individ- ual lumps of constant sizes, it follows that the moners are to be regarded as the natural bodies which effect the transition from inorganic to organic nature. The following list of publica- tions gives the literature of the important dis- coveries in this field in chronological order : Hugo Mohl, Ueber die Saftbewegung im In- nern der Zellc (in Botanitche Zeitung, 1846); Ferdinand Cohn, Nachtrdge zur Naturge- tchichte des Protococcu* plunialis (in Nora Acta Natures Curiotorum, 1850); Hugo Mohl, Grund- euge der Anatomic und Physiologic der tege- tabilwchen Zelle (1851) ; Franz Unger, Anato- mic und Physiologic der Pflamen (1855) ; Max Schultze, Innere Bewegungserscheinungen lei Diatomeen (in Troschel's Architftir Naturge- tchichte, 18(50), Die Gattung Cornuspira unter den Monothalamien, &c. (1860), and Ueber Muskelkorperchen und das was man eine Zell eu nennen habe (1861) ; Ernst BrQcke, Elemen- tar-Organism (in Sitzungsberichte der Wiener Akademie, 1861); Ernst Haeckel, Die Sareodc der Radiolarien : Monographic der Radiola- rien (1862); Lionel Beale, " The Structure of the Simple Tissues of the Human System" (1862); Mai Schultze, Das Protoplatma der Rhizopoden und der Pflamentellen (1863); Haeckel, Ueber den Sarcodekorper der Rhizo- poden (Zeittchrift fiir wissenschaftliche Zo- ologic, 1864); Wilhelra KQhne. Untersuchun- gen uoer das Protoplastma und die Contractili- tdt (1864); Haeckel, Oenerelle Morphologic der Organismen (1866), and Monographic der Mo- neren (in Jena ische Zeitschrtftfur Naturwissen- tchaft, 1867); Huxley, "Protoplasm, or the Physical Basis of Life " (1868), and " On some Organisms living at Great Depths in the North Atlantic Ocean " (in " Journal of Microscopical Science," 1868); Haeckel, Beitrdge *ur Phtsti- den Theorie (in Jenaiwhe Zeitxchrift, 1870) ; Rudolf Virchow, Die Cellularpathologie in ihrer Begrundung auf physiologische und pa- thologische Gewebelehre (4th ed., 1871) ; Edou- ard van Beneden, Recherches ur revolution det gregarines (in Bulletin de Facademie royale de Belgique, 1871) ; Haeckel, Monographic der Ealkschw&mme (1872) ; James Ross, " On Pro- toplasm " (London, 1874) ; John Drysdale, M. D., " The Protoplasmic Theory of Life " (1875) ; and H. Charlton Bastian, " Evolution and the Origin of Life" (1875). "As regards Proto- plasm," by J. H. Stirling (Edinburgh, 1869), is intended as a refutation of the theory. PROTOZOA (Gr. Trporo?, first, and ov, ani- mal), 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 animal- cules, as well as the large sponges. They are composed of a nearly structureless, jelly-like substance, called protoplasm or sarcode, with- out distinct segments, internal cavity, or ner- vous system, and with no or a very rudimen- tary digestive apparatus. (See PROTOPLASM.) Dr. Engelmann has observed in arcella, a mi- nute amoeba-like protozoan, a periodical devel- opment of gas in the granular protoplasm, un- connected 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 ab- sorption were not determined. The usually accepted division is into the classes of gregari- nidce, rhitopoda (like amcebao, foraminifers, and sponges), and infusoria, the highest, with a mouth and digestive apparatus, like the bell animalcules and paramctcium. As these rep- resent the first step in animal organization, so do the protophytes tho first in vegetable life ; the former were called ouzoa by Carus, from their resemblance to the ova or germs of higher animals; the latter, as far as known, were microscopic seaweeds, without the radi- ate structure characteristic of plants, and are found in the lower Silurian strata. (See ANI- MALCULES, BATHYBIUS, COCCOLITHS, FORAMI- NIFERA, GLOBIOERINA, and GREQARINA.) See Prof. Packard on tho " Development of Pro- tozoa," in tho " American Naturalist," Decem- ber, 1874, to February, 1875. PROTRACTOR, an instrument for laying off angles in plotting. There are four principal forms of the protractor : the rectangular, the semicircular, the circular, and the reflecting. The rectangular consists usually of a thin rec- tangular piece of ivory or metal, three edges of which are graduated from to 180 degrees by portions of radii converging to the middle of the fourth edge as a centre ; it is used only where a loose approximation to accuracy suf- fices. The circular and semicircular protractors, with either two, one, cr no arms, are graduated circular arcs (usually metal), with or without flat straight-edged arms, turning about their perforated centres, and carrying verniers for the accurate reading of their arcs. But as they are only capable of protracting and measuring single angles on a map, they have not so wide a. range of usefulness in engineering and sur-
