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4 site to the disease may therefore be said to be complete.

There are several species of malaria parasites. The distinctive characteristics of each of these will be detailed in Chap. III. The following brief description is confined to what may be regarded as the generic features common to all.

The three phases.—The malaria parasite, like all true parasites, must be adapted not only for a life inside its hosts, but also, in order that its continuance as a species may be assured, for a passage from one host to another. Consequently, as regards man, it exhibits two distinct phases—an intracorporeal

Fig. 1.—Evolution of the tertian parasite, unstained (see text).

and an extracorporeal. Clinical observation and analogy make it certain that there is yet another phase, also intracorporeal—the latent phase, whose characters as yet can only be conjectured.

Each species of malaria parasite has its special and more or less definite intracorporeal life-span or cycle of twenty-four hours, of forty-eight hours, or of seventy-two hours.

On examining microscopically malarial blood towards the end of one of these cycles, an hour or two before the occurrence of a paroxysm of the characteristic periodic fever it induces, the parasite may be