254 THE SPOROZOA 



so distinguished. The essence of being a parasite is not to reproduce 

 sexually, but to flourish at the expense of other creatures, and the term 

 " host " denotes the being that suffers in proportion as the parasite profits. 

 Of the two hosts of the malarial parasite there can be no question that in 

 this sense also the Vertebrate is the principal one, since the mosquito appears 

 to suffer scarcely at all It certainly would not be in the interests of the 

 parasite that the vitality of the mosquito should be lowered and its 

 appetite impaired. 



Those who term the Vertebrate the intermediate host of the malarial 

 parasite, do so chiefly on the analogy of parasitic worms, Cestodes or 

 Trematodes, in which the sexual stages are passed in the definitive host, 

 the larval stages in an intermediate host. But the relation between 

 schizont and sporont can scarcely be considered analogous to that between 

 Cercaria and Distomum, for example. The comparison should be rather 

 with the summer and winter generations of Aphis or Daphnia, or with 

 Hydroid and Medusa. 



The variations in the structure and life -history of other 

 Haemosporidia, as compared with the type here selected, are best 

 considered, as was done in Coccidia, first from the point of view of 

 the morphology of the individual stages, secondly from that of the 

 life-cycle considered as a whole. 



(1) Morphology. The trophozoites of Haemosporidia may be 

 distinguished, speaking generally, either as "haemamoebae" or as 

 " haemogregarines." Those parasitic upon cold-blooded Vertebrates 

 are not amoeboid like the malarial parasites, but have a fixed body- 

 form like minute gregarines (Figs. 75-77). This is true not only 

 of the " free " phases, but also of the endoglobular forms. They 

 occur generally as tiny vermicules, lodged in the blood-corpuscle or 

 free in the blood-plasma. When free they are often very active 

 in their movements, bending and twisting their bodies from side 

 to side, or gliding forwards in the manner already described for 

 Gregarines or Coccidian sporozoites (pp. 180 and 210), by the help 

 of a secreted thread of gelatinous substance (Hintze [68]). The 

 fixity of the body-contour seems to be due to the dense hyaline 

 ectoplasm, in which myocy te-fibrillae can often be made out. The 

 haemogregarines vary greatly in size, in different genera, relatively 

 to the dimensions of the blood -corpuscles they attack. Thus, 

 while Lankesterella scarcely attains to half the length of the frog's 

 blood-corpuscle which it inhabits (Fig. 75), the species of Haemo- 

 gregarina parasitic in various reptiles grow to such a length that 

 in later stages the trophozoite becomes folded on itself in a 

 characteristic manner, in order to be packed away within the 

 limited space at its disposal (Fig. 77). The genus Piroplasma, on 

 the other hand, is characterised by pear-shaped trophozoites of 

 extremely small size, several of which may be lodged in a single 

 blood-corpuscle (Fig. 80). 



