THE SPOROZOA 165 



be considered from two points of view, according as they affect 

 the characters (I.) of the individual stages or (II.) of the whole 

 life-cycle. 



I. Each phase of the life-history may be varied or modified in 

 structural or other details, in accordance with the special environ- 

 ment and conditions of life to which a given species of these 

 parasites is adapted. The modifications that occur under this 

 head will receive detailed treatment in due course in the systematic 

 review of the orders, families, and genera of Sporozoa in the sequel, 

 but a few of the more important variations and simplifications may 

 be considered here. The trophozoites have commonly, as in 

 Monocystis, a definite body-form, limited by a cuticle ; but in many 

 forms the protoplasm is naked, and the body is amoeboid, and of 

 indefinite and changeable form. In Monocystis the gametes are not 

 differentiated and the conjugation is isogamous, but in other types 

 there may be anisogamous conjugation between sharply differentiated 

 male and female gametes. The greatest variation, however, is 

 seen in the spores. The number of the sporozoites is usually 

 eight in Gregarines, but may be greater or less in other types. 

 Hence the spores are distinguished as monozoic, dizoic, tetrazoic, 

 polyzoic, and so forth, according as they contain one, two, four, or 

 many sporozoites. In the monozoic condition there is no secondary 

 multiplication within the sporocyst, but each sporoblast simply 

 becomes a sporozoite. In many such cases the sporoblast secretes 

 no sporocyst, but becomes a naked gymnospore, resembling a free 

 sporozoite. These gymnospores may be formed within a resistent 

 cyst secreted round the sporonts, or the cyst may be entirely 

 absent. And further, the spore -formation may be preceded by 

 conjugation of gametes, or the spores may be produced asexually, 

 by the segmentation of a single sporont. 



Hence in an ideally primitive type of sporozoan development, 

 the full-grown trophozoite or sporont simply breaks up, without 

 previous conjugation or encystment, into a number of naked 

 gymnospores, and each of them becomes a trophozoite which is 

 similar to its parent, and repeats the process in due course. 



II. The plan and character of the life-cycle as a whole may 

 be greatly varied, and secondary modifications or complications of 

 various kinds introduced into it. The more important of these 

 variations will be briefly described. 



(1) In Monocystis it has been seen that the period of growth 

 and the period of proliferation are sharply separated, the latter fol- 

 lowing upon the former. The same is the case in the whole order 

 Gregarinida, to which Monocystis belongs, and also in two other 

 orders, the Coccidiidea arid the Haemosporidia. On the other 

 hand, in the two orders known as Myxosporidia and Sarcosporidia, 

 spore-formation commences at an early stage in the growth of the 



