166 THE PROTOZOA 



Sporocysts are often simply rounded or oval bodies, like cysts, 

 but in some cases they exhibit special forms, and may be prolonged 

 into spikes, tails, or processes of various kinds. 



In many cases the purely protective uses of the cyst may be 

 combined with the performance of some special function within it. 

 The contained organism may remain merely in a resting state 

 within the cyst (hypnocyst) ; or it may utilize its leisure for the 

 digestion of large quantities of ingested food - material, or for 

 carrying on processes of reproduction or syngamy. As a process 

 of similar nature to encystment, the formation of " sclerotia " in 

 the Mycetozoa must be noted (see p. 240, infra). 



Active adaptation to changed conditions is seen in those forms in 

 which the mode of life is bound up with changes of environment 

 during different periods of the life-history that is to say, more 

 especially in parasitic forms, in which a change of hosts is necessary 

 for the continuance of the species. In such forms there are in 

 general two functions for which provision must be made : the first 

 is that of multiplying in the host itself and keeping up a stock of 

 the parasites in it ; the second is that of infecting a new host sooner 

 or later (see p. 20, supra). In the most primitive types of para- 

 sitic Protozoa there is no differentiation of form or structure corre- 

 sponding to these two distinct functions ; but as a general rule 

 a given parasite in a given host exhibits usually two forms or series 

 of forms, which may be termed " multiplicative " and " propagative " 

 respectively (Doflein). Multiplicative forms may be wanting in 

 some cases, as in the Eugregarines, but propagative forms are 

 always found, being an absolute necessity for the continuance of 

 the species. 



As examples of multiplicative and propagative forms, we may 

 consider first species which are parasitic only on a single host in 

 the course of the entire life-cycle. A typical example is seen in 

 Coccidium (p. 342, Fig. 152), in which adult forms, " schizonts," 

 multiply rapidly in the host by a process of multiple fission, " schi- 

 zogony," a process which takes place unaccompanied by any sexual 

 phenomena, and in which no resistant cysts are formed, since they 

 are quite unnecessary. Sooner or later, however, generations of 

 individuals, " sporonts," appear which do not multiply like the 

 schizonts, but which, as gametocytes, give rise to the gametes. After 

 a process of syngamy the zygote forms a resistant cyst within 



to a developmental cycle ; the " spores " of Bacteria are for the most part simply 

 cysts, but are called spores on account of their small size. 



In this book the word "spore, "when not qualified by any prefix, will be used 

 to denote a resistant seed-like body protected by a tough envelope, or sporocyst, 

 and the production or development of such bodies will be termed " spore-forma- 

 tion." On the other hand, the production of numerous small cells or germs by 

 multiple fission will be termed " sporulation." 



