180 PARASITISM 



is usually limited to simple division or budding, although spore forma- 

 tion is occasionally met with here (e. g., noctiluca, colpidium, etc.), 

 while after fertilization spore formation is quite common, especially 

 among the free flagellates. In parasitic forms, on the other hand, 

 and especially among sporozoa, simple division and budding are 

 extremely rare, being replaced here by the more prolific multiple 

 reproduction by asexual spore formation in response to the greater 

 need of numbers in maintaining the species. Two kinds of "spores," 

 therefore, may be present in these parasitic protozoa, the one giving 

 rise to infection of new hosts (spores s. str.), the other to auto-infection 

 of the same host. No little confusion has arisen because of this dif- 

 ference, and various writers have sought to avoid it by giving different 

 terms to the "spores" of varied origin. Such efforts, instead of help- 

 ing, have, in the main, made "confusion more confused," and students 

 of the group have recognized the need of adopting some one standard 

 and acceptable terminology. At the present time there is a tendency 

 to eliminate the term "spore" as applying to any definite reproductive 

 body, and to reserve it for a general designation of any reproductive 

 body formed in brood. Specialists, however, especially those dealing 

 with the sporozoa, have generally applied the term in a still more 

 limited sense to the reproductive bodies in gregarinida and coccidiidia 

 within the sporocysts which give rise to the sporozoites or final repro- 

 ductive elements (Fig. 74). Such a young sporozoite as that of Cocci- 

 dium (Eimerid) schubergi grows into a vegetative organism termed a 

 trophozoite, which finally becomes a schizont and reproduces asexually, 

 forming spores known as merozoiies (Fig. 74, c). These reproductive 

 bodies are naked and unable to withstand the unfavorable exigencies 

 of an external life, but are capable of developing within the same 

 host. They, too, grow into trophozoites, and the process of schizogony 

 may be repeated many times; ultimately, however, vitality wanes and 

 the organisms become sexually mature. The trophozoites, at this 

 period, instead of forming schizonts, turn into gametocytes and give 

 rise to conjugating gametes, which may or may not be sexually dif- 

 ferentiated. The gametes conjugate and form a zygote or copula 

 which becomes a sporoblast or by division gives rise to sporoblasts. 

 The sporoblasts are enclosed in protective coatings termed sporocysts, 

 and within these they multiply again to form from two to many 

 germs, the sporozoites, or the sporoblast may, in some cases, become 

 the sporozoite directly without further division. The various forms 

 assumed by the sporozoan parasites and the many kinds of repro- 

 ductive bodies bring about great complexity in the life cycle, and 

 where only one phase of such a cycle is known, confusion is apt to 

 follow attempts at classification. 



There is no doubt that the group of sporozoa which furnishes some 

 of the best and most complete examples of the life cycle of protozoa 



