THE SPOROZOA 239 



ORDER 3. Haemosporidia. 



The Haemosporidia are a group of Sporozoa adapted to a very 

 special mode of parasitism, and therefore limited in habitat and 

 occurrence. They exhibit, however, an interesting range of 

 variations, both in morphological structure and in adaptation to 

 their special life -conditions. Their distinctive features are as 

 follows. They are parasitic usually upon the red blood-corpuscles, 

 sometimes also upon other cells, of Vertebrata. The trophozoite is 

 endoglobular, i.e. intracellular, in situation, and may remain so 

 throughout the whole trophic period, or may quit the host-cell 

 and become free in the blood-plasma after reaching a certain stage 

 of growth. The endoglobular forms are very commonly amoeboid, 

 but those which become free have definite body-contours, and 

 resemble tiny gregarines of elongated form and worm-like appear- 

 ance, which when liberated from the blood-corpuscle are actively 

 motile. The life-cycle shows an alternation of generations similar 

 to that occurring in Coccidia. In all cases, probably, non-sexual 

 reproduction by means of schizogony continued through many 

 generations serves to multiply the parasites within the host, and 

 is then followed by the formation of gametes, which conjugate to 

 produce zygotes or sporonts. Each sporont is at first motile, and 

 seeks out a suitable position in which to become encysted as an 

 oocyst. It then undergoes sporogony to form a number of minute 

 germs, which are always naked gymnospores or sporozoites, never 

 enclosed in sporocysts. In many forms, perhaps in all those 

 parasitic upon warm-blooded animals, the entire sexual cycle takes 

 place in an intermediate host, an invertebrate animal of blood- 

 sucking habits, upon which the sporont is actively parasitic, and 

 by which fresh vertebrate hosts are inoculated with the germs of 

 the parasite. 



Our scientific knowledge of the Haemosporidia is of extremely recent 

 date, and begins with the discovery, by Lankester, in 1871, of the 

 parasite of the frog's blood, which in 1882 he named Drepanidium 

 ( = Larikesterelki) ranarum, and recognised as a member of the Sporozoa. 

 At the latter date Laveran, then a military doctor at Constantin in 

 Algiers, discovered the malarial parasite in human blood. He described 

 all its characteristic stages amoebula, rosette, crescent, sphere, and 

 flagellated body and saw in it the cause of the disease, but it was many 

 years before his ideas became generally accepted. Laveran did not at 

 first recognise the true nature of the parasite he had discovered, but 

 regarded it as a vegetable organism and named it Oscillaria malariae. 

 Metschnikoff was the first to place it amongst the Sporozoa, under the 

 generic designation Haematophyllum (1887); it had already, however, 

 been named Plasmodium by Marchiafava and Celli in 1885. Our know- 

 ledge of these organisms was further advanced by the studies of Danilewsky, 



