THE HAEMOFLAGELLATES 245 



forms originally parasitic in Invertebrates ; in other words, the 

 Invertebrate was regarded as the primary host, the Vertebrate as 

 the secondary or intermediate one. We now think this view was 

 probably, to a considerable extent, wrong ; in this we have been 

 mainly influenced, on the one hand, by the intestinal Trypanoplas- 

 mata, and on the other, by the case of T. grayi. As above remarked, 

 it seems evident that a Vertebrate is the primary host of this latter 

 parasite ; and the same would follow, by inference, for the other 

 (Mammalian) Trypanosomes transmitted by Tsetse-flies. Moreover, 

 the writer thinks he did not allow sufficient weight to the fact 

 that the Invertebrates which harbour Trypanosomes are, with but 

 few exceptions, blood-suckers. For these reasons we are now inclined 

 to consider most of the Invertebrates concerned (e.g. leeches, many 

 biting-flies, etc.) as the secondary, intermediate hosts of various 

 Vertebrate parasites (probably all the Piscine and Amphibian ones, 

 many, but perhaps not all the Mammalian ones, and perhaps some 

 Avian ones). 



The only important 1 exceptions are among Insects ; and here 

 it is quite likely that we have both primary and secondary hosts. 

 Besides the Tsetses, Tabanids, etc., the common house-fly and 

 related genera, in which Herpetomonads (e.g. H. muscae-domesticae, 

 H. sarcophagae, etc.) occur, ought apparently also to be placed in 

 the category of secondary hosts. For Prowazek (I.e.) states 

 that, according to Brauer, the latter flies are probably de- 

 scended from blood-sucking ones; in Avhich case their parasites 

 may very well be descended from haemal forms, which are now, 

 perforce, restricted to the Invertebrate host. On the other hand, 

 there are several instances of the parasites occurring either in non- 

 sanguivorous Insects or in forms that only rarely suck blood, 

 which are, we think, more likely cases of primary parasitism. 

 Among these, for example, are H. bombycis, in Bombyx mori ; H. 

 gracilis, in larvae of Tanypus ; Crithidia campanulata, in larvae of 

 CMronomus plumosus. Lastly, we have the mosquitoes and their 

 parasites, both of imago and larva. The latter is, of course, 

 aquatic, and the imago is by no means limited to blood for nutri- 

 ment. Having regard also to the illustrative series of transitional 

 forms between the monadine type and the trypaniform one, made 

 known by Leger and others, it appears to us that here as well the 

 Insect is the primary host of the various Flagellates concerned, and 

 that where these are connected with a Vertebrate host the latter is 

 to be regarded as the secondary, intermediate one. This would 

 apply chiefly to certain parasites (e.g. Trypanomorpha) of birds, 

 though not necessarily, it is to be noted, to all. 



Many authorities (such as Laveran and Mesnil, Liihe, Novy and 



1 Herpetomonas biitschlii from a Nematode (Trilobus) and the curious Trypano- 

 phis from Siphonophores do not appear to have any bearing upon this question. 



