188 PARASITISM 



and rhizopods of the digestive tract of different animals, and is well 

 illustrated by the case of Co'promonas siibtilis, a parasite of the frog 

 (Fig. 66, p. 153). Here two complete individuals are fused into one, the 

 copula forms a chitinous cyst and passes with the feces to the outside. 

 No multiplication takes place within the cyst, and infection of a new 

 host is brought about by feeding. A somewhat more complicated 

 history is presented by the intestinal amebse, where encystment and 

 fertilization (in these cases autogamous) is followed by the formation 

 of spores, usually in small numbers, which are not liberated until the 

 definitive seat of parasitism is reached. Here, again, although several 

 young may be formed at the period of fertilization, there is apparently 

 little reason to imagine any great difficulties to be overcome by the 

 parasites in finding a new host. 



Since, in flagellates, amebse, and sporozoa, encystment is thus bound 

 up with fertilization, it would not be unreasonable to argue that where 

 such cysts are found, previous fertilization may, at least, be suspected. 

 Too much importance must not be attached to encystment, however, 

 for in many forms, especially in the free flagellates, ciliates, and rhizo- 

 pods, encystment may be brought about by the temporary adverse 

 condition of the surrounding medium, or even for purposes of diges- 

 tion. The encysted trypanosome, Trypanosoma grayi, which Minchin 

 ('07) discovered in the rectum of the tsetse fly, Glossina palpalis, 

 may be due to such change in the medium, or, which is less probable, 

 may be interpreted as a result of fertilization. This is the only case 

 among trypanosomes in which an encysted stage has been noted, 

 although Moore and Breinl ('07) have described small reproductive 

 bodies in Trypanosoma gamhiense, which may have a like significance. 

 In this case, however, they are found in the blood and belong obviously 

 to the endogenous cycle (see p. 267). Metcalf ('07) has shown that 

 encystment of Opalina intestinalis and dimidiaia which occurs in the 

 rectum of the frog, has nothing to do with conjugation. The cysts 

 pass out with the feces and after a longer or shorter period may again 

 be taken into a tadpole's digestive tract, where, after dissolution of the 

 cyst, a larger macrogamete fuses with a smaller "tailed" gamete, so 

 that fertilization in this case follows encystment. (INIetcalf does not 

 find conjugation between two " tailed" forms, as Neresheimer describes, 

 see p. 1.58.) 



It is among the sporozoa that the most remarkable and best- 

 illustrated phenomena of exogenous sporulation are to be found, and 

 here there is almost every conceivable grade of complexity. Owing 

 to the heterogeneous nature of the sporozoa and the wide variations 

 in the processes of sporogony, confusion must follow any attempt to 

 describe them all in one category. Generalizations can be made only 

 in connection with the more homogeneous groups of gregarinida and 

 coccidiidia, while the hemosporidia and other parasitic forms will 



