84 



ZOOLOGY 



SECT. 



organs, most frequently in those of the epithelium of the digestive canal. They 

 never inhabit blood-corpuscles. A few are intra-nuclear parasites. Two 

 distinct modes of multiplication occur — by schizogony, a kind of multiple fission, 

 and by sporogony, a process of spore -formation preceded by conjugation between 

 male and female cells. The trophozoite, or adult phase, as we may term it, of 

 the parasite, grows to a certain size within the cell without destroying its 

 vitality — the nucleus merely being pushed on one side. So far, in fact, from 

 impairing the nutrition of the cell, the presence of the parasite seems, in some 

 cases, for a time, rather to stimulate it At a certain stage of growth schizogony 

 (Fig. 65, 6 — c) takes place. The nucleus divides to form a number of nuclei. 

 These migrate towards the surface, and each becomes surrounded by protoplasm, 

 with the result that a number of small cells are formed. Each of these gives 

 rise to a club-shaped merozoite. The merozoites, when they become free, are 

 active bodies, which are able to penetrate into the interior of other epithelial 

 cells and develop into trophozoites like those from which they were derived. 

 This multiplication may take place on such an extensive scale that the 



IE;. 



S.Coccidium 



Fig. 64.— Coccidlidea. A, adult JSimcria (e) in enteric epithelial cell (qi.) of mouse ; 

 B, encyatedform ; C, encysted fomi, the protoplasm contracting to foi-niaspore ; D, foi-mation 

 of falciform young (/.) in interior of spore (sp.); B, spore with falciform young; F, adult 

 encysted form of Cocr></iinti from liver of rabbit ; G, division into spores ; H, cyst containing 

 ripe spores (.sj).), each vfith a single falciform young ; I, single spore with falciform young (/.). 

 (Fi-om Biitschli's Protozoa, after Leuckart and Eimer."\ 



epithelium may be partially or completely destroyed. It is only, apparently, 

 when such extensive damage has been done, or is threatened, that multiplication 

 by sporogony takes place — the invasion of a new host being by this process 

 rendered probable, and the continuance of the race being thus provided for in 

 the event of the death of the host in which the epithelium has become destroyed. 

 In this process certain of the merozoites, instead of developing into trophozoites, 

 grow more slowly (d), and become converted into either micro- or megagame- 

 tocytes. Each of the former (A, j) gives rise by division to a number of narrow 

 biflagellate microgametes or sperms. Each of the megagametocytes (e, /), after a 

 process of the nature of maturation, forms a single rounded megagamete (ovum). 

 When this becomes fertilised by the penetration into it of a single microganiete, the 

 resulting body {zygote or oo.iperm) divides to form a varying number of cells 

 each enclosed in a resistant cyst {k) . These give rise to spores with a firm, chitinous 

 spore-membrane, each containing two or more falciform young or sporozoites [I). 

 The cyst destroys the cell as it grows, and thus becomes free in the cavity by 

 which the epithelium is lined. The spores may thus pass out to the exterior, 

 and, if taken into the digestive canal of a new host, may liberate the now 

 active sporozoites, which may penetrate into epithelial cells (a) to become the 

 trophozoites with which the cycle began. 



