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 in Senegambia in 1901, in a European suffering from intermittent fever. Forde discovered the parasites, but was uncertain of their nature; he shewed them to E. Dutton, who (11) gave this form the name of Trypanosoma gambiense. A year later A. Castellani (6) found the organisms (most probably the same species) in the cerebro-spinal fluid of patients suffering from sleeping-sickness in Uganda; and it has since been conclusively proved by Sir David Bruce and D. Nabarro (4) that they are the true cause of that dreadful malady.

More important from the standpoint of protozoology, than these interesting medical discoveries have been the investigations by A. Laveran and F. Mesnil (20–24), L. Léger (30–35), S. Prowazek (47), F. Schaudinn (50) and others, upon numerous tolerated (i.e. non-pathogenic) forms; these researches supply, indeed, practically all the material facts on which to base an account of the Haemoflagellates at the present day.

Trypanosomes are harboured by members of all the chief classes of vertebrates with the exception of cyclostomes. By far the greater number of hosts are furnished by fishes, birds and mammals. Among batrachians the parasites have been found, up till now, only in frogs; and among reptiles their occurrence has only been observed in one

or two solitary instances (T. damoniae, fig. 3 J). Data with regard to the frequency with which individual species occur, in any kind of host, are as yet somewhat scanty; in one or two cases the parasites are fairly common, T. lewisi, for example, being met with in a considerable percentage of sewer-rats throughout the world.

In considering the occurrence of Trypanosomes in mammals, careful distinction must be drawn between natural or true hosts, which are tolerant of the parasites, and casual ones, which are unaccustomed and unadapted to them. A Trypanosome usually produces markedly harmful effects upon gaining an entry into animals which have never been, by their distribution, liable to its invasion previously. Such a state of affairs is produced by the march of civilization into the “hinterlands” of the various colonies, when man, together with the numerous domesticated animals which accompany him, is brought into proximity to big game, &c., and, what is equally important, into the zone of the particular blood-sucking insects which prey upon the same.

Very many of the common domestic mammals can be successfully infected (either thus accidentally or else on purpose) with different “pathogenic” Trypanosomes, to which they succumb more or less readily, but they cannot be regarded as the natural hosts of those Trypanosomes. In dealing with disease-causing forms, the more narrowly the original source of the parasite concerned is defined, the closer do we get to the true vertebrate host or hosts. In the case of the nagana-parasite, various Antilopidae (e.g. the gnu, bushbuck and koodoo) can certainly lay a strong claim to the honour. The capybara, again, is most probably the native host of T. equinum of mal de caderas of horses in South America. Similarly with regard to the many other pathogenic Trypanosomes now known, there is undoubtedly, in each case, some indigenous wild animal tolerant of that particular form, which serves as a “latent source of supply” to strange mammals.

The transmission of the parasites from one vertebrate individual to another is effected, in the great majority of cases by a blood-sucking invertebrate, and by this means minion; alone. The “carrier” of a Trypanosome of warm-blooded vertebrates is, in all instances so far described, an insect, generally a member of the Diptera; in the case of parasites of cold-blooded vertebrates the same rôle is usually played by an ichthyobdellid leech (piscine forms), but possibly, now and again, by an Ixodes (amphibian or reptilian forms).

Until lately it remained quite uncertain, however, whether the invertebrate merely conveys the Trypanosomes or whether it is a true alternate host, one i.e. in which definite stages of the parasite's life-cycle are undergone. Schaudinn (50), who investigated certain avian Trypanosomes, considered the latter View to be correct, and believed, that the carrier—in this instance a gnat—is indeed the definitive host, i.e. the one in which sexual conjugation occurs. Many other workers have since studied the subject and, so far as the parasites of fishes are concerned, there can be little doubt, thanks to the researches of E. Brumpt (5a), L. Léger (32, 33) and others, that leeches are true alternate hosts for these forms, in which certain phases of the life-cycle are normally undergone.

We cannot write quite so confidently with regard to the relation of the various pathogenic Trypanosomes to Tsetse-flies (Glossinae). In the first place experiment has shown that biting-flies, other in all probability than the true, natural hosts, may at times transmit the parasites—as it were—accidentally, if, after feeding on an infected animal, they are allowed to bite a fresh one within a limited time. One very helpful factor in determining which is the principal carrier of any form is the coincidence of the zone of a particular insect with that of any disease. By this means it has been ascertained with practical certainty that, among the family of Tsetse-flies (Glossinae) for instance, at least four species are the natural carriers of different Trypanosomes. Of these perhaps the best-known is G. palpalis, of Equatorial Africa, whose bite transmits the human parasite (T. gambiense). Nevertheless, the fact, commented upon by several observers, that even here an infected fly is only infectious fora comparatively short period suggests that this species of fly, at any rate, is not the true alternate host in which the life-cycle of that particular Trypanosome is completed. However, indications furnished by Koch (16a) point in this connexion to G. fusca. Lastly, before leaving this interesting and important subject, F. Stuhlmann's work (54a) on developmental phases of T. brucii, the nagana parasite in G. fusca and G. tachinoides, does render it probable that the pathogenic forms also have true invertebrate hosts.

Schaudinn had fully described the relations of certain avian Trypanosomes to their invertebrate host, Culex pipiens (females). The distribution of the parasites in the gnat is closely connected with the process of digestion. The Trypanosomes ultimately overrun practically all parts of the body, sometimes not even the ova escaping. Thus true hereditary infection of a succeeding generation of gnats may be brought about. The life of the parasites while in the insect is characterized by an alternation of active periods, during which multiplication goes on, with resting-periods, when the Trypanosomes become attached to the epithelial cells” of the host. According to S. Prowazek (47), the behaviour of T. lewisi in a louse (Haematopinus) is, in its main features, similar.

On gaining an entry into the blood of a vertebrate the organisms pass rapidly into the general circulation, and are thus carried all over. Considering them first in a tolerant host, the trend of observation is to show that they are never abundant, but on the contrary usually somewhat scarce. One reason for this scarcity is to be sought in connexion with the fact that multiplicative stages are very rarely met with, at any rate in the general circulation. The parasites are frequently more numerous in the spleen, bone-marrow, kidneys, &c., than elsewhere, and it has been found that multiplication goes on rather more actively in the capillaries of these organs.

The Trypanosomes, in the active phase, are of course always free in the blood plasma (interglobular). In the majority of cases it is very uncertain whether they actually come into relation with the blood corpuscles or not. Schaudinn has stated, however, that Trypanomorpha becomes, in certain phases, attached to a red blood-corpuscle (ectoglobular), and, in others, penetrates inside one and eventually destroys it (endog1obular); while his other avian parasite, Trypanosoma ziemanni, apparently draws up into itself the white corpuscle (leukocyte) to which it becomes attached. In addition, there are two or three