48 ENCYSTMENT OF THE PEOTOZOA 



must occur, it is the encysted forms which render such a transference to 

 a new host possible. In the case of the intestinal amoebse, though both 

 free and encysted forms escape from the body, it is only the encysted 

 stages which are able to carry infection to a new host. Even if direct 

 transference of unencysted stages occurred, these would, in all probability, 

 be killed by the digestive fluids of the stomach, which the encysted stages 

 can withstand. In the large group of insect flagellates (Trypanosomidse), 

 again, it is by means of minute encysted stages passed in the faeces that 

 another insect is infected (Fig. 164). 



When encystment is about to occur, very frequently changes take 

 place in the encysting organism. The cytoplasm is freed from all food 

 residues, and in consequence becomes much clearer. Not infrequently 

 the cytoplasm becomes charged with food reserve material, such as 

 glycogen. Sometimes, as in the case of Entamoeba coli and Entamoeba 

 histolytica, in preparation for encystment special forms of the amoeba 

 which are smaller than the ordinary free individuals are produced (Figs. 

 96 and 100). These forms, which are preparing for encystment, are known 

 as precystic forms. Amongst the Sporozoa encystment is associated 

 with a sexual process. In the case of the gregarines two individuals 

 become enclosed in a cyst (gametocyst), within which each gives rise to a 

 number of gametes (Fig. 465). The gametes unite in pairs, and the 

 zygote thus produced itself becomes encysted in the oocyst, within which 

 it divides into a number of sporozoites. In the case of the coccidia, the 

 zygote is encysted in the oocyst which is formed either before or after 

 syngamy has taken place (Fig. 337). Within the oocyst the zygote divides 

 into a number of sporoblasts, which in their turn become encysted in 

 sporocysts. Inside the sporocysts the sporoblasts divide into sporozoites. 



A special type of cyst is produced by the Cnidosporidia. These are 

 provided with one or more polar capsules from which long filaments can 

 be rapidly extruded. They serve the purpose of anchoring the cysts in 

 the intestine, while the wall is opened for the liberation of the enclosed 

 organism (Fig. 311). 



For a long time the resistant encysted stages of the Cnidosporidia, 

 coccidia, and gregarines were known as psorosperms, a name introduced 

 by Johannes Miiller (1841) for the spores of Myxosporidiida. The spindle- 

 shaped oocysts of gregarines were frequently referred to as pseudo- 

 navicellse, a name first used by von Siebold (1839). 



The production of secondary cysts within the primary one occurs 

 occasionally in other groups, as in the ciliates of the genus Colpoda. A 

 ciliate may become encysted and undergo a diminution in size within the 

 cyst, and then form a second cyst. The process may even be repeated 

 again, giving rise to three concentric cysts. As stated above, the different 



