134 LIFE-HLSTORY OF PROTOZOA 



The latter unite in pairs to form the zygotes, which give rise to typical 

 daughter individuals, known as sporozoites. These grow into adults, 

 which reproduce repeatedly in the manner characteristic of the repro- 

 ductive phase till certain of the progeny again become gametocytes. 

 The various forms which occur during the multiplicative phase, which is 

 known as agamogom/, belong to the asexual generation, while the indi- 

 viduals themselves are agamonts. In contrast to these, the gametocytes 

 and the gametes to which they give rise, the zygotes, and the sporozoites 

 which are ultimately formed, belong to the sexual generation. The 

 process of development from gametocyte to sporozoite is known as 

 sporogonij, while the gametocytes themselves are sporonfs. These two 

 phases of development alternate in that, after reproduction has been 

 repeated a number of times (agamogony), the sexual method of multiplica- 

 tion (sporogony) supervenes. The sequence of the two phases is known 

 as alternation of generations, which is a characteristic of the majority of 

 the Sporozoa. Amongst the typical gregarines, however, the asexual 

 generation and agamogony does not occur, the sporozoites into which the 

 zygotes divide growing directly into gametocytes, which again produce 

 gametes. The whole life-cycle of a typical gregarine is thus one of 

 sporogony. 



As already indicated, the life-cycle of a Protozoon may at any time 

 be interrupted by the formation of protective cysts secreted from the 

 ectoplasm. Some organisms cease multiplying when they become en- 

 cysted, others continue to multiply within the cyst, while others again 

 never reproduce except in the encysted condition. Sometimes, as in the 

 case of parasitic amoebse, special individuals (precystic amoebse) alone 

 are capable of forming cysts. Amongst the Sporozoa, encystment only 

 occurs in association with sporogony. A cyst may be formed around 

 two gametocytes, as in the case of gregarines. It is then distinguished 

 as a gametocgst. After syngamy has taken place, the resulting zygote 

 may secrete a cyst known as an oocyst. The zygote may divide into a 

 number of sporoblasts, and these, either within the oocyst or after their 

 escape from it, become enclosed in secondary cysts called sporocysts. 

 Oocysts and sporocysts occur typically amongst the Sporozoa. The cysts 

 usually have very tough and resistant walls ; at other times they are little 

 more than thin membranes. 



Protozoa may be free-living organisms which spend the whole of their 

 life in water or in moist situations, or they may be more or less intimately 

 associated with other animals. According to the degree of this dependence 

 three classes are usually recognized. There are commensals, which live 

 in or upon another organism, and, though deriving benefit from this 

 association, do not injure the host in any way. They deprive it of an 



