THI-: H^MOFLAGELLATES. 161 



to prove that other forms agree with the examples men- 

 tioned above^ at least so far as regards the broad features of 

 their life-history. For instance; with respect to the question 

 of their unity in possessing an alternation of true hosts^ the 

 Trypanosomes are at the moment^ in a position quite similar 

 to that in which, until hitely, the Hasmosporidia were. 

 It has been customary hitherto to sharply separate the 

 Hfemosporidia of cold-blooded from those of warm-blooded 

 Vertebrates, notwithstanding their close agreement in habi- 

 tat and morphology, on the ground that the former had 

 no alternation of hosts. Recently, however, Siegel (105) 

 demonstrated such an occurrence in the case of Haemo- 

 gregarina stepanovi, parasitic in a tortoise, the discovery 

 being at the same time extended by Schaudinn for another 

 member of the Hasmosporea, namely, Karyolysus 

 1 ace rt arum. The alternate hosts, in which in both cases 

 the sexual process is undergone, are respectively a leech 

 and a tick.- Hence it may be said with practical certainty 

 that a definitive Invertebrate host is common to all the 

 Hsetnosporidia, and that being so the distinction between 

 the two sub-orders vanishes. The arguments in favour of a 

 similar fundamental agreement in the case of the different 

 Trj'panosomes may be discussed under two principal head- 

 ings : — (1) the important and, in fact, essential part of trans- 

 mitter of the parasites played by a blood-sucking Inverte- 

 brate, which is in some cases known, and strongly surmised 

 in others, to be a true alternate host; and (2) the evidence 

 v/hich points unmistakably to a close connection between, 

 at any rate, certain Hajmotlagellates and certain Ha^mo- 

 sporidia. The various observations, etc., coming under each 

 heading are, however, considered in detail below (see espe- 

 cially section 9). 



' Since this was written Prowazek (88) lias described in detail the Hfe- 

 cycle of T. lewisi. Many of the phases, including sexual conjugation, are 

 undergone in a louse (Hiematopinus), which is a true alternate host for 

 this parasite. This important discovery helps to bring Mammalian Try- 

 panosoraes into line with the rest. 



- Placobdella catenigena and Ixodes ricinus. 



