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X] account of some of the corresponding infections of certain of the lower animals, such as, when the opportunity offers, might supply him with the necessary material for study. If infected horses, oxen, dogs, etc., are not always available, the rat is to be met with everywhere, and can be relied on in most places to afford the student opportunities for acquiring much useful knowledge about trypanosomes, as well as experience in their microscopical diagnosis.

It must be remembered that T. brucei, T. gambiense, and T. evansi exhibit a wide range of pathogenicity for many species of animals.



Our knowledge of the trypanosomes is still incomplete. Any attempt, therefore, at classification must be merely tentative. Bruce's classification of the pathogenic African species (p. 194), founded on very large experience in the field, has the merit of being of practical value. It is based mainly on three points—morphology, location of development of infective form in the tse-tse fly, and action on animals. Bruce divides the parasites into three groups—(A) T. brucei group, (B) T. pecorum group, (C) T. vivax group.

Of the trypanosomes of mammals the best known and most important are the following (Fig. 52):—

T. lewisi (Kent, 1879) is a parasite of rats (Mus decumanus, M. rattus, M. rufescens). Similar parasites in hamsters, guineapigs, rabbits, etc., probably belong to different species; it has been held that T. lewisi is not capable of living in these animals, but subsequent work seems to contradict this.

On account of the facility with which it can be procured in most countries, T. lewisi offers the best opportunity for study of the genus. In many places from 2 to 50 per cent. of the local rats harbour it, often in such abundance that, viewed