THE H^MOFLAGELLATES AND ALLIED FORMS 291 



some of the sheep, it would furnish another instance of hereditary 

 transmission. Hence this mode of transmission must, apparently, 

 be reckoned with in some instances, though it is evidently an ex- 

 tremely rare phenomenon in trypanosomes generally. 



Just as a given species of trypanosome is, in Nature, capable of 

 maintaining itself only in a particular species, or limited group of 

 species, of vertebrate hosts, so it may be said, as a general rule, 

 that in transmission by the cyclical method the parasites are 

 specific in the same way to certain invertebrate hosts, in which 

 alone they are able to go through their full natural cycle. Amongst 

 the many blood- sucking invertebrates which may prey upon the 

 vertebrate, we may distinguish " right " and " wrong " hosts ; in 

 the right host or hosts the parasite establishes itself more or less 

 easily, and passes through a full and complete developmental 

 cycle ; in the wrong host it either dies out immediately or goes 

 through only a part of its cycle. The distinction between right 

 and wrong hosts must not, however, be taken in an absolute sense, 

 but as implying only that, amongst many possible hosts, there is 

 one at least to which the parasites have become better adapted 

 than to any other ; but the trypanosomes may sometimes succeed 

 in maintaining themselves in other than the right host sufficiently 

 long to pass back again into the vertebrate. Thus, in the case of 

 the rat-trypanosome (T. lewisi) the right host is a rat-flea (Cerato- 

 phyllus fasciatus, or possibly other species) ; but it may persist in 

 the rat-louse (Hcematopinus spinulosus), and even pass from it, 

 though rarely, back into the rat again. 



The following are a few well-established examples, in addition 

 to that of T. lewisi already cited, of trypanosomes and their right 

 hosts. Many pathogenic species of trypanosomes in Africa are 

 transmitted by tsetse-flies e.g., T. gambiense and T. vivax by 

 Glossina palpalis, T. brucii by G. morsitans* etc. The recently - 

 described T. cruzi of Brazil was discovered in its invertebrate host, 

 a blood-sucking hemipterous insect, Gonorhinus megistus, before it 

 was found in the blood of human beings. The trypanosomes of 

 certain fresh-water fishes namely, goldfish, perch, etc. pass 

 through their developmental cycle in the leech Hemiclepsis mar- 

 ginata (Robertson, 503). T. mice of skates and rays develops in 

 the leech Pontobdella muricata (Robertson, 500, 502). The trypano- 

 some of African crocodiles, T. grayi, develops in the tsetse-fly 

 Glossina palpalis (Kleine, 458 ; Kleine and Taute, 459), and stages 

 in its life-cycle have consequently been confused with those of 

 T. gambiense in the same fly. The trypanosomes of birds are prob- 

 ably transmitted for the most part by mosquitoes, but the details of 



* According to Taute, G. morsitans can act as a true host for T. gambiense, and, 

 conversely, according to Fischer, 0. palpalis can do the same for T. brucii. 



