142 



THE PROTOZOA 



bore into the epithelial cells, where they grow (Fig. 77). All forms, 

 apparently, begin life as intra-cellular parasites, where, at first, they do 

 little harm, but as they grow by the absorption of fluids contained 



within their cell-hosts, the latter 

 are improperly nourished and, 

 unless the parasites leave them, 

 they degenerate and die (Fig. 78). 

 The duration of intra-cellular life 

 varies in different kinds of Spo- 

 p rozoa : some are permanently 

 intra-cellular (monophagoiis forms, 

 so-called Cytosporidia, etc.); others 

 are intra-cellular only in the young 

 or immature phases(Gregarinida); 

 while still others pass different 

 phases of their life-history in dif- 

 ferent cells (polyphagous forms). 

 " The mature parasites finally may 

 leave the cell-host and sporulate 

 *' ' :: ^T ~' ''""****' in the digestive cavity or coelom, 



Fig. 78. Coccidia in the epithelial cells of J . . 



Triton cristatus, n, nuclei of the tissue cells; and the spores are then carried 



>, the intra-cellular parasite Pfeifferia tritonis. to t h e outside with the faeCCS Or 



TLABBE '- ] 



other excreta. 



A. PROTOPLASMIC STRUCTURE 



A typical sporozoon consists of protoplasm and one nucleus. It 

 has no mouth, anus, excretory pore, or other openings. It has 

 neither gastric nor contractile vacuoles, and has at most a sluggish 

 movement in the adult stage, although the young forms may be 

 amoeboid or flagellate. Owing to the number of cytoplasmic granules 

 which make up the bulk of the animal, the protoplasmic structure of 

 adult forms can be made out only with the greatest difficulty. Apart 

 from these granules, however, which are regarded as reserve nutri- 

 ment, it is probable that, as in all Protozoa, the protoplasm is alveolar. 

 This is certainly the case in Coccidiida (intra-cellular Sporozoa), espe- 

 cially in the young forms, where Labbe ('96) describes the cytoplasm 

 as alveolar; in some forms of Gregarinida (Fig. 87), and in Myxo- 

 sporidiida as described by Thelohan ('95) and Doflein ('98) (Fig. 79). 

 The granules, which are so characteristic of the group, completely 

 fill the alveolar network, and give to the protoplasm its peculiarly 

 dense appearance. They differ somewhat in size and shape, and 

 apparently in chemical composition, and are generally regarded as 

 food substances reserved for use during the spore-producing period. 



