PROTOZOA. Fundamentally contrasted with all other types of animal life, which are together designated the Metazoa, in that the body consists of a single cell or a colony of simple cells, the group is still usually termed a phylum though it is evidently of a higher grade than the subdivisions of the Metazoa to which that rank is always assigned and the designation sub-kingdom seems better justified. The organ isms included are all minute; the largest being barely visible to the naked eye and the smallest lying at or beyond the limits of optical vision. With great simplicity of structure they com bine functional complexity equal in a general way to that in the Metazoa. Although for the most part the group is well defined yet there exist forms that constitute an unbroken series from such as manifest clearly animal character istics to those which are equally certainly plants. Transitional forms that bridge the gap between Protozoa and Metazoa are also well known; these are colonial types in which a differentia tion between certain cells marks the first step in the production of the Metazoan organism. While structurally simple the Protozoa have many specialized parts, the cell-organs or or ganellae, for the discharge of particular func tions. Thus for locomotion one finds projec tions of the external layer, either in the form of soft, mobile protoplasmic lobes that undergo constant changes in form, the pseudopodia, or in fixed processes of contractile substance known as flagella and cilia that carry out vibrating movements which propel the body and aid in taking food. The simplest types con sist, like the Amoeba, of a mere drop of naked protoplasm enclosing a nucleus. In most forms, however, a cell-membrane covers the mass and imposes evident limits on its activity, though it is on the whole an advance in specialization.
Many forms possess protective coverings like shells, scales and spines that are permanent, or cysts which are of a temporary character and related to special conditions such as repro duction, drought and seasonal changes. Mem braneless forms ingest food at any point but those species with a membrane have at least a mouth opening and sometimes a second orifice for the extrusion of solid waste. The mem brane may form at the mouth an inturned fun nel-shaped esophagus. Many parasitic species have an unbroken membrane and take nourish ment by osmosis. Contractile vacuoles fed by delicate radiating canals serve to eliminate waste and excess of water from the protoplasm, thus serving both as respiratory and as excretory organs. The existence of anuclear forms, the Monera of Heeckel, is now strongly On the other hand the normal mononuclear con dition is varied by a binuclear type in some groups. One also finds a multinuclear condi tion which may indicate a colonial organism or a phase in reproduction just antecedent to spore formation. Division is the ordinary method of reproduction and may be simple or multiple either in the free or the encysted state. Similar spores (isogametes) or dissimilar spores (micro- and macro-gametes) are produced by most Protozoa, and may develop asexually, but in many cases fuse by pairs as a preliminary phase in the life cycle that suggests the start of Metazoan development.
Many parasitic species exhibit the alterna tion of two different generations, or a change of hosts as a characteristic factor in their life history. Parasitic Protozoa may infect an organ or an individual cell, or even be found in a nucleus; and the various phases in the life cycle may illustrate these different locations as the malarial plasmodium is successively a cell parasite in the red corpuscle, an organ parasite in the blood, and a tissue parasite in the wall of the mosquito's midgut. No less distinctly
than in Metazoa are the effects of parasitism seen among Protozoa as in the degeneration of structures characteristic of free life, in the formation of hold-fast organs, in the increase of reproductive activity, in the occurrence of metagenesis, and in the adjustment of the parasitic life cycle to the habits of the host. Protozoan parasites are the cause of serious and even fatal diseases when by tissue destruc tion they produce ulcers or by tissue tion they give rise to tumors. Metabolic prod ucts resulting from the activity of the invading Protozoa may exert a powerful toxemic influ ence on the host as is well recognized in malaria for instance. The source of human Protozoal diseases may be found in the inges tion of encysted spores with impure food or drink, or it may be traced to the active inocula tion of such spores through the medium of some blood sucking insect, like the mosquito, bedbug or louse. The control of Protozoal diseases de pends clearly upon knowledge of the life cycle of the organism and of its means of transmis sion. The brilliant successes of recent years in controlling malaria, yellow fever and other Protozoal diseases depended absolutely on at tacking the parasite at some weak point in its life cycle and I.% eventing its dissemination putting an end to the disease. In this great work American investigators have played an important role. The most important Protozoal diseases of mai, arc amcrbic dysentery due to 'several species of Entamceba; sleeping sickness of tropical Africa caused by Trypanosoma, a flagellate blood parasite; malaria, a scourge of tropical and subtropical regions since history -began, produced by Plasmodium. Inyellow fever the organism is yet undiscovered but ap parently belongs in the Protozoa; and in typhus it is a minute spirillar structure some times placed here and sometimes included under the Protdphyta. To other less important human diseases must be added many formidable pests •to which domestic animals are subject and a host of others that attack all sorts of animals. These facts serve to show the great and grow ing importance of Protozoa in an economic way. Among the Protozoa are distinguished the following major sub-divisions, termed 'classes or less often phyla: (1) Sarcodina, naked forms moving by pseudopodia; (2) Mastigophora, usually with a membrane and with flagella for locomotion; (3) Sporozoa, exclusively parasitic, without locomotor proc esses of any sort. Spores without polar capsule; (4) exclusively para sitic, sometimes amceboid. Spores with one or more polar capsules; (5) Infusoria, with firm membrane, moved by cilia. Among groups in terior sedis are the Spirochwtida, often in cluded near the Mastigophora, and the Chlamy dozoa, possibly related to the Sarcodina; the first contains the organism of syphilis, the sec ond those of some obscure diseases, but the exact place of both is at the present date very doubtful. Consult Calkins, G. N., 'Protozoa> (New York 1901) ; Lankester, E. R., 'A 'Treatise on Zoology' (London 1903) ; Hartog, M. M., 'Protozoa' (in 'Cambridge Natural History,' Vol. I, New York 1909) ; Minchin, E. A., 'Introduction to the Study of the Protozoa' (New York 1912).