THE SPOROZOA 271 



Comparison of the Life-Cycles of the Telosporidia. 



It is evident that the Haemosporidia resemble the Coccidia 

 very closely in all essential points. Their life -cycle can be 

 described in identical terms, and the points of difference are 

 mainly adaptive. They might, in fact, be considered simply as 

 Coccidia adapted to parasitism upon a special form of cell, the 

 blood-corpuscle, as has been done by Mesnil (see p. 229, footnote). 

 This point of view is not, however, strictly accurate, as the 

 Haemosporidia exhibit certain features, not obviously correlated 

 with their mode of life, which are not seen in Coccidia. Such 

 are (1) the frequent occurrence of amoeboid phases in the growth 

 of the trophozoites ; (2) the free, extra -cellular gregarine-like 

 forms characteristic of one sub-order ; (3) the occurrence of schizo- 

 gony by simple binary fission ; and (4) the motile " vermicule " 

 phase of the zygote. Some of the above characters, notably 

 (1) and (3), are clearly of a primitive nature, and could easily be 

 explained as an inheritance from an ancestor common to them 

 and to the Coccidia. 



The two orders Haemosporidia and Coccidia may therefore be 

 regarded as two very closely allied groups of the Telosporidia, 

 which have diverged from a common origin in two directions, in 

 accordance with the difference in their habitat. Doflein has 

 recently given expression to this view by placing the Coccidia 

 and the Haemosporidia as two sub-orders of a single order, the 

 Coccidiomorpha. 



On the other hand, the exact homologies between the different 

 stages of the life -cycles of the Gregarinida and Coccidiomorpha 

 respectively are not so obvious, and require brief discussion. In 

 both groups the life-cycle may be complicated by schizogony, but 

 for purposes of detailed comparison it is necessary to eliminate all 

 secondary or adaptive phases of development, and to select types 

 in which the life-history runs the simplest course. In other words, 

 the comparison must start from the consideration of a mono- 

 genetic type of development by sporogony, such as is found in 

 the vast majority of Gregarines, and in Benedenia amongst Coccidia. 



In a typical Gregarine such as Monocystis, or, better still, 

 Stylorhynchus, the life-cycle may be formulated as follows : 



In a monogenetic Coccidian, the life -cycle may be expressed 

 thus : 



