154 H. M. WOODCOCK. 



longitudinal fission, but multiple division or segmentation is 

 also met Avith. The complete life-history, where known, is 

 ver}^ complicated. It includes true bi-sexual conjugation, 

 which takes place in the Invertebrate; and it appears very 

 likely that, in most instances at &uy rate, this host is to be 

 considered as the definitive and primary one, and the Verte- 

 brate as the intermediate or secondary one. 



Section II. Introductory. 



A Study of Recent and Rapid Clrowth. — Even more 

 marked than in the case of the Sporozoa has been the recent 

 great and rapid increase in our knowledge of the Trypano- 

 somes. The bulk of the important research on these organisms 

 has been accomplished, indeed, within the last four or five 

 years, culminating, for the time being, in the remarkable 

 and far-reaching discoveries announced by Schaudinn at 

 the beginning of 1904. The realisation of the extreme 

 economic importance of these parasites is mainly responsible 

 for this advance. Until almost the commencement of the 

 present century they had been very little studied from a 

 purely zoological point of view. Apart from the work of 

 Danilewsky in the eighties scarcely anything had been 

 previously done towards elucidating their morphology and 

 life-history. Reasons are not far to seek which explain, at 

 any rate, to a certain extent, this lack of interest. 



Occurrence. — The minute size of the parasites, together 

 with their habitat in the blood, renders them, unlike the 

 majority of Sporozoa, very inconspicuous; and they are 

 consequently overlooked unless specially searched for. In 

 tiie light of recent investigation, however, it cannot be 

 maintained that ^ri-ypanosomes are at all limited in distribu- 

 tion. Tor, although they are restricted,^ so far as is known, to 

 blood-sucking Insects and leeches among Invertebrates, they 



' The allied form Trypanopli is is an exception, being parasitic in 

 certain Siplionopliora. * 



