r R o p R o Spencer resolves protoplasm into "physiological units," Haeckel into "plastidules," while Darwin accounts for heredity by reference to the properties of supposed " gem- mules," Engelmann suggests the existence of "contractile units " (isotaymen), &c. ; but those various hypotheses, framed mostly for special purposes, still await more general criticism. See (6). 8. Origin of Protoplasm. See ABIOGENESIS, BIOLOGY, REPRODUCTION, and (7). Bibliography. In addition to the articles above referred to, the reader may with advantage consult the following works, from which complete bibliographical details can be obtained. (1) For general history see Sachs, Gcschichtc d. Botanik ; Cams, Ge- schichte d. Zoologic; Engelmann, " Physiol. d. Protoplasma u. Flimmerbewegung," in Hermann s Handb. d. Physiologic, i. , Lcipsie, 1879 ; and for special history, Dujardin, " Recherches s. 1. organismes inferieurs," especially iii., " S. 1. pretendus estomacs d. animales Infusoires ets. u. snbst. appelee Sarcode," Ann, d. Sci. Nat., 1835, p. 367; H. v. Mohl, " Ueb. d. Saftbewegung im Innern d. Zelle," Bot. Zcitunrj, 1846, p. 73; Max Schultze, Ueb. d. Organismus d. Polythalamicn. Leipsic, 1854 ; Id., "Ueb. Mus- kelkbrperchen," &c., Arch. f. Anat. u. Physiol., 1861; Id., D. Protoplasma d. Rhizoj>oden u. d. Pflanzenzellcn, Leipsic, 1863 ; De Bary, "Die Myectozoen," Zcitsch. f. icisscnsch. Zool., x., 1859 ; Haeckel, Gcnerelle Morphologic, i., Berlin, 1866. (2) For general structure and properties see Hofmeister, D. Lehrc d. Pfl. -Zelle, Leipsic, 1867 ; Sachs, Lchrb. d. Botanik, or Engl. translation, Oxford, 1882 ; Sachs, Vorlesungcn iib. Pflanzcn- Physiol. , Leipsic, 1882 ; and other text-books of vegetable physiology, especially those of Reinke and Pfeff er ; Strasburger, D. Botan. Pradiatm, Leipsic, 1884 ; and Engelmann, op. cit. Among special works see Kiihne, Untcrsuch. iib. d. Prolop. u. d. Contractilitdt, Leipsic, 1864 ; Strasburger, "Studien lib. d. Protoplasma," Jena. Zcitsch., x., 1876, also Ueb. Zcllbildung u. Zelltheihmg, 2d ed., Jena, 1876. (3) Dohrn, Das Principd. Fimctionswcchscls, Leipsic, 1875; Foster, Handbook of Physiology, 4th ed., London, 1884. (4) For ex perimental researches see Engelmann, Kiihne, and other authors cited under (2) passim ; also Darwin, Insectivorous Plants, London, 1876; Frommann, "Ueb. Structur, Lebenserschcin, &c., d. Thier. u. Pfl.-Zellen," Jena. Zcitsch., xviii., 1884. (5) Gamgee, Physiol. Chem. of Anim. Body, London, 1880 ; Foster, Handbook of Physi ology ; Gaxitier. Chimie appliquee d la Physiologic ; Schiitzenberger, " Recherches s. 1 Albumine, &c.," Bull. d. I. Soc. Chimique, 1875 ; Leow and Bokorny, Biol. Central- Blatt, i., No. 7 ; and Reinke, " Ueber d. Protoplasma," in Untcrsuch. a. d. bot. Lab. Univ. Gdttingcn, 1881. (6) Spencer, Principles of Biology, vol. i. ; Haeckel, Gen. Morph., i. ; Darwin, Variation under Domestication; Engelmann, op. cit. (7) Engelmann, op. cit. (P. GE.) PROTOZOA PROTOZOA is the name applied to the lowest grade of the animal kingdom, and originated as a translation of the German term " Urthiere." Whilst at first used some forty years ago in a vague sense, without any strict definition, so as to include on the one hand some simple organisms which are now regarded as plants and on the other some animals which are now assigned a higher place in the animal series, the term has within the last twenty years acquired a very clear signification. The Protozoa are sharply and definitely distinguished from all -the rest of the animal kingdom, which are known by the names " Metazoa " or " Enterozoa." They are those animals which are structurally single " cells " or single corpuscles of protoplasm, whereas the Enterozoa consist of many such units arranged definitely (in the first instance) in two layers an endoderm or enteric cell-layer and an ectoderm or deric cell-layeraround a central cavity, the enteron or common digestive cavity, which is in open communication with the exterior by a mouth. The Protozoa are then essentially unicellular animals. The individual or person in this grade of the animal king dom is a single cell ; and, although we find Protozoa which consist of aggregates of such cells, and are entitled to be called " multicellular," yet an examination of the details of structure of these cell-aggregates and of their life- history establishes the fact that the cohesion of the cells in these instances is not an essential feature of the life of such multicellular Protozoa but a secondary and non-essen tial arrangement. Like the budded "persons" forming, when coherent to one another, undifferentiated " colonies " among the Polyps and Corals, the coherent cells of a com pound Protozoon can be separated from one another and live independently ; their cohesion has no economic signifi cance. Each cell is precisely the counterpart of its neigh bour; there is no common life, no distribution of function among special groups of the associated cells, and no cor responding differentiation of structure. As a contrast to this we find even in the simplest Enterozoa that the cells are functionally and structurally distinguishable into two . ,TOups those which line the enteron or digestive cavity and those which form the outer body wall. The cells of these two layers are not interchangeable ; they are funda mentally different in properties and structure from one another. The individual Enterozoon is not a single cell ; it is an aggregate of a higher order consisting essentially of a digestive cavity around which two layers of cells are disposed. The individual Protozoon is a single cell; a number of these individuals may, as the result of the pro cess of fission (cell-division), remain in contact with one another, but the compound individual which they thus originate has not a strong character. The constituent cells are still the more important individualities ; they never become differentiated and grouped in distinct layers differing from one another in properties and structure ; they never become subordinated to the individuality of the aggregate produced by their cohesion ; hence we are justified in calling even these exceptional aggregated Protozoa unicellular. By far the larger number of Protozoa are absolutely single isolated cells, which, whenever they duplicate them selves by that process of division common to these units of structure (whether existing as isolated organisms or as constituents of the tissues of plants or of animals), separ ate at once into two distinct individuals which move away from one another and are thenceforward strangers. Whilst it is easy to draw the line between the Protozoa and the Enterozoa or Metazoa which lie above them, on account of the perfectly definite differentiation of the cells of the latter into two primary tissues, it is more difficult to separate the Protozoa from the parallel group of unicellular plants. Theoretically there is no difficulty about this distinction. There is no doubt that organisms present themselves to us in two great series starting in both cases from simple unicellular forms. The one series, the plants, can take up the carbon, hydrogen, oxygen, and nitrogen necessary to build up their growing protoplasm from mineral com pounds soluble in water, compounds which constitute the resting stage of those elements in the present physical conditions of our planet. Plants can take their nitrogen in the form of ammonia or in the form of nitrates and their carbon in the form of carbonic acid. Accordingly they require no mouths, no digestive apparatus ; their food being soluble in water and diffusible, they absorb at all or many points of their surface. The spreading diffuse form of plants is definitely related to this fact. On the other hand the series of organisms which we distinguish as animals cannot take the nitrogen, necessary to build up their protoplasm, in a lower state of combination than it 1 presents in the class of compounds known as albumens; nor can they take carbon in a lower state of combination
than it presents when united with hydrogen or with