376 THE PROTOZOA 



different type. In these cases the invertebrate host appears to be always 

 an ectoparasitic arthropod. The only life -cycle of such forms which has been 

 described completely is that of the parasite of the leucocytes of rats, which 

 has been described by Miller under the name Hepatozoon pemiciosum. This 

 parasite appears to be identical with that named by Balfour (694) Leucocyto- 

 zoon muris and by Adie L. ratti ; its correct name, therefore, is Hcemogregarina 

 (Hepatozoon) muris. According to Miller, this parasite causes lethal epidemics 

 amongst tame rats, but in London it occurs commonly in the blood of wild 

 sewer- rats, and appears to be quite harmless to them. It is a parasite of world- 

 wide distribution, apparently, having been recorded from rats in the Punjaub 

 (Adie), Khartoum (Balfour), North America (Miller), Brazil (Carini), and various 

 other parts of the world (see Franca and Pinto, A.I.B.C.P., iii., p. 207). 



The life-cycle of H. muris, according to Miller, is in the main as follows: 

 The sporozoites are liberated in the intestine of the rat, and pass through the 

 wall of the gut into the blood-stream ; they may be found in the circulation 

 twenty-four hours after infection. Ultimately the sporozoites reach the 

 liver .and penetrate into liver-cells ; in this situation they grow into schizonts, 

 which when full-grown sporulate to produce some twelve to twenty, usually 

 about sixteen, merozoites. The merozoites may penetrate into liver- cells 

 again and repeat the schizogony, or they may pass out into the capillaries 

 of the liver ; in the latter event they are taken up by leucocytes, doubtless 

 as an act of phagocytosis. The merozoites are able, however, to resist any 

 digestive action of the leucocytes ; they become encapsuled in the leucocytes, 

 and in this state they are carried into the general circulation. They do not 

 increase in size in the leucocytes, and their further development, so far as the 

 rat is concerned, is at an end. Hence the " leucocytozoon " of the rat is an 

 encapsuled merozoite of a haemogregarine which, strictly speaking, is a para- 

 site of the rat's liver, and not of the blood at all ; in the leucocytes its role is 

 one merely of passive resistance. These merozoites represent at the same time 

 the sporonts, the propagative phase which develops further in the inverte- 

 brate host, in this case a rat-mite, Lcelaps echidninus, which sucks the rat's 

 blood, and so takes up the parasite into its stoim.ch. 



In the stomach of the mite the haemogregarines are set free as motile vermi- 

 cules which associate in couples. According to Miller, this association is a 

 true copulation of two gametes which fuse into a zygote ; from the analogy 

 of the life-cycle described above, it is more likely that some stages have been 

 overlooked, and that the vermicules are gametocytes which associate, with 

 subsequent production of gametes by the male and fertilization of the female 

 by a microgamete. 



The zygote, however formed, becomes a motile ookinete which passes 

 through the wall of the gut into the body- cavity of the mite, and there forms 

 an oocyst which, like that of the malarial parasites, has a thin wall, permitting 

 the parasite to absorb nourishment from the surrounding tissues and to grow 

 to a large size. When full-grown, the contents of the oocyst divide up into a 

 large number of sporoblasts, each of which becomes surrounded by a delicate 

 sporocyst. The contents of the spore divide up into some twelve to twenty 

 sporozoites, and then the development of the parasite is at an end so far as 

 the mite is concerned. The cyst and spores are the propa'gative phase, and 

 in order t^hat they may develop the mite must be eaten by a rat ; if this occurs, 

 the sporozoites are liberated in the stomach and the cycle is complete. 



In the case of other mammalian haemogregarines, fragments of the develop- 

 ment are known which indicate a life-cycle similar in the main to that of 

 H. muris, allowing for specific differences. Forms parasitic in the red blood- 

 corpuscles are H. gerbitti of Gerbittus indicus (Christophers, 699) ; H. bed f our i 

 (jaculi) of the jerboa (Balfour, 693) ; and the three species recently described 

 by Welsh and others (Journ. Path. Boot., xiv.) from marsupials, one of which 

 (H. peramelis) is remarkable for having been found only in the free, extra- 

 corpuscular condition. The schizogony of H. gerbilli has not been described, 

 but that of H. jaculi takes place in the liver, and is of two types, producing 

 in the one case a large number of small merozoites, in the other a small 



