240 THE HAEMOFLAGELLATES 



of the medium in the Insectan host ; on the contrary, we consider 

 that the former, whatever indications it may furnish, cannot replace 

 altogether the latter. 



It seems to us that Novy and M'Neal entirely fail to appreciate 

 the intimate and specific relations of Protozoan parasites to their 

 hosts, and the remarkable degree to which their biology is adapted 

 to the same. The Sporozoa in their entirety illustrate this, so do 

 other parasitic Protozoa, and there is no reason to suppose the Haemo- 

 flagellates are different. We agree fully with Brumpt that the 

 chemical and physiological medium of a particular Invertebrate is 

 essential for the adequate development of all such phases of the 

 life -history of a Trypanosome as may be undergone outside the 

 Vertebrate host. And the various researches above summarised, 

 which go to show that there are right and wrong hosts for the 

 parasites, and that only certain " ripe " phases, the outcome of the 

 sojourn in the right host, can reinfect the Vertebrate host success- 

 fully, afford strong support to this view. 



Another criticism put forward by Novy and M'Neal and others 

 is that the Flagellate phases found in the mosquitoes (Culex), which 

 Schaudinn regarded as belonging to Trypanomorpha of the Little 

 Owl, were in all likelihood purely Insectan parasites, of a Herpeto- 

 monad or Crithidial type, which had nothing to do with the blood 

 forms. Before discussing this view it is necessary to consider 

 briefly the subject of these Insectan Flagellates, one which is also of 

 very great importance because of its bearing upon the phylogeny 

 and derivation of the Trypanosomes. 



7. THE INSECTAN FLAGELLATES : THE EVOLUTION AND 

 PHYLOGENY OF TRYPANOSOMES. 



(a) The Insectan Flagellates. 



Several of the earlier workers have commented upon the occur- 

 rence of Flagellates in mosquitoes. Thus in 1898 Ross observed 

 parasites which he has recently (74) compared with Leger's genus 

 Crithidia in Anopheles, larva, pupa, and imago. A similar parasite 

 was found by Christophers in 1901, occurring in swarms in 

 Anopheles and Culex. Durham, again, the year before, had 

 noticed numerous " Trypanosomes " in a Stegomyia which had fed 

 upon a bat. The first serious contributions, however, to our know- 

 ledge of the Flagellates parasitic in Insects are Leger's researches 

 (47, 48, 51, 52), 1902-1904, on certain Herpetomonadine forms. 



Besides the genus Herpetomonas, Leger has distinguished 

 another type of form, which he has termed Crithidia. Both types 

 show, in general, an alternation of monadine (flagellate) phases 

 with gregariniform (resting, non- flagellate) ones. In the latter 

 condition, the parasites occur as small, rounded, pear-shaped, or 



