278 H. M. WOODCOCK. 



lewisi among Mammalian forms, T. ziemanni among Avian 

 parasites, and T. n el spr intense among Amphibian types. 

 And the same is probably true of the majority of the Piscine 

 forms ; at any rate, their morphology — the only evidence 

 available at present — agrees more closely with that of Mam- 

 malian parasites than with that of Trypanomorpha. On 

 the other hand, in the case of one or two Amphibian forms, 

 it is much more doubtful which interpretation of their 

 morphology is the correct one. One of these, most unfor- 

 tunately, happens to be Trypanosoma rotatorium, the 

 type-species of this genus. It can at least be said, however 

 that this parasite is quite as probably a Trypanoplasmatine 

 form as a Monadine one (see below, p. 288). 



Evolution. 



In conclusion, one may, perhaps, venture to indicate what 

 appears to have been the general course followed in the 

 evolution of the Htematozoa, although the tentative cha- 

 racter of any hypothesis which can be at present advanced 

 must be fully recognised. 



The Hgemoflagellates are here considered to be, in general, 

 descended from forms originally parasitic only in Invertebrate 

 hosts (probably non-predatory), in Avhich, as intestinal or 

 entero-coelomic parasites, all their life-cycle was undergone. 

 Upon their becoming associated with a blood-sucking host, 

 either by the acquirement by the original one of this mode of 

 life, or by the adaptation of a particular parasite to a closely 

 allied predatory host, the Flagellates gradually became 

 specialised for life in the blood. In other words the Inver- 

 tebrate is here regarded as the primary host, and the 

 Vertebrate as the secondary or intermediate one. 



There can be no doubt that this is the case in Trypano- 

 morpha, at any rate. In this form the development of the 

 characteristic undulating membrane appears to have been 

 brought about by this change in habitat. Transitional stages 

 are beautifully illustrated by Leger's observations (I.e.) on Her- 



