PROTECTIVE AND REPRODUCTIVE CYSTS 47 



in those of Dimastig amoeba gruberi (Fig. 120). To prevent drying of the 

 contents of the cyst, such pores are closed by plugs of some material 

 formed by the cytoplasm. They probably facilitate emergence from the 

 cyst. 



Those Protozoa which are able to contract during life to the spherical 

 form produce spherical cysts, but others become encysted without changing 

 their shape to any extent. Thus, the species of Giardia produces ovoid 

 cysts, while species of Chilomastix and Embadomonas cysts which are pear- 

 shaped (Figs. 293, 255 and 256). The cysts (oocysts) which are formed 

 round the zygotes of various species of Eimeria are frequently ovoid in 

 shape, while those which form round the zygotes of the Gregarinina are 

 generally spindle-shaped (Figs. 350 and 483). Though the majority of 

 Protozoa form cysts at some stage of their development, there are some 

 forms in which cysts have never been observed. 



The behaviour of the organism within the cyst varies considerably. 

 In many cases the cysts are purely protective in nature, the organism 

 remaining unchanged in the cyst till circumstances again become favour- 

 able to a free existence. The encysted organism escapes from the cyst 

 by its gradual dissolution, or through special pores when these are present. 

 In other cases multiplication takes place within the cyst. In the case of 

 Entamoeba coli, for instance, the nucleus of the encysted amoeba divides 

 repeatedly to produce eight nuclei (Fig. 101). Within the oocysts of 

 coccidia and gregarines there are produced a varying number of daughter 

 individuals known as sporozoites (Fig. 337). Similarly, within the oocysts 

 of the malarial parasites on the stomach of the mosquito there are de- 

 veloped very large numbers of sporozoites (Fig. 391). Within the cyst 

 of Giardia there are produced two daughter flagellates, while in that of 

 Prowazekella lacertw as many as sixty-four daughter flagellates are formed 

 (Fig. 253). Amongst the ciliates, when cysts are formed, they are usually 

 purely protective in nature, but in some cases, at least, reproduction 

 within the cyst takes place. Thus, the various species of Colpoda appear 

 to reproduce only in the encysted condition. The ciliate becomes spherical, 

 and by constant rotation forms a spherical cyst. Within it division into 

 two and then into four daughter ciliates occurs. The cyst is then ruptured 

 and the four young ciliates emerge. They then grow into the adult form, 

 when the process is repeated (Fig. 38). 



Cyst formation is a very characteristic feature of parasitic Protozoa. 

 Having adapted themselves to life within another organism, their powers 

 of survival under external conditions have been largely lost, and it is b^ 

 means of their encysted stages that they are able to pass from one host 

 to another. It thus arises that whenever an organism passes from one 

 host to another in such a manner that exposure to external conditions 



