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Rh parasites, so far as it has a bearing on human pathology. As to classification, still a quœstio vexata, the one adopted in Prof. Minchin's " Introduction to the Protozoa " (1912) has been for the most part followed.

Protozoa are unicellular organisms possessing a nucleus distinct from the cytoplasm. They belong to the Protistenreich of Haeckel, a kingdom which unites the simplest and most primitive forms of life, and which may be considered equivalent in systemic value to the animal and vegetable kingdoms. The protozoal cell constitutes an entire individual; it may exist singly, or combined in the form of cell colonies.

The body is a mass of protoplasm sometimes enclosed in a limiting envelope. Such organs as are present are also protoplasmic, and are used for the purposes of locomotion and capture of food. Metabolism is generally of the animal type.

Reproduction takes place by fission. Sexual reproduction generally occurs throughout the group, and, as compared with other Protista, the developmental cycle may be a very complicated one in which the same organism reappears in a totally different form at different periods of its development.

There are certain forms of development in the blood protozoa which it is necessary to define and for which certain terms are used. Their application, however, when loosely used has given rise to much confusion. The protozoa parasitic in blood of the vertebrate host undergo a vegetative or non-sexual multiplication, the process being then known as schizogony; but in order to ensure their further existence, and their passage from one vertebrate host (intermediary host)* to another, a further cycle in a cold-blooded invertebrate host (definitive host) is necessary, during which the sexual cycle or sporogony takes place.

In the vertebrate blood the young parasite is known as a schizont. The products of the asexual fission are known as merozoites; these merozoites at some period of their existence develop into forms known as sporonts, destined to pass into the alimentary tract of the inveitebrate host. The term trophozoite