74 Reproduction and Life-Cycles 



of apparent functions, protective and reproductive cysts have been dis- 

 tinguished. 



Protective cysts may be developed directly from active stages, from 

 zygotes in Volvox and Gregarinida, or from sporoblasts (division-products 

 of the zygote) in Coccidia. Such cysts usually possess rather firm walls 

 (Fig. 2. 15, AD), the composition of which varies from group to group. 



Fig. 2. 15. A. Cyst of Ceratium liiruncUnella; x385 (after Hall). B. Pro- 

 tective cyst of Didinium nasuhim; outer (ectocyst) and inner (mesocyst) 

 membranes evident; x310 (after Beers). C. Protective cyst of Bursaria 

 truncatella; xl35 (after Beers). D. Encysted zygote of Volvox globator; 

 diagrammatic (after Janet). E. Reproductive cyst in Gyrodinium sp.; x240 

 (after Kofoid and Swezy). F. Reproductive cyst in Colpoda citcuUus; x735 

 (after Kidder and Claff). Key: c, chromatophore; e, developing ectocyst; 

 71, nucleus in syncytial layer enclosing zygote; zn, nucleus of zygote. 



The cyst membranes of many ciiiates are probably composed largely of 

 proteins (172), although the inner meinbrane (endocyst) may be carbo- 

 hydrate in nature (100). In Endamoebidae and Giardia, the properties 

 of the cyst wall resemble those of keratins (155). Siliceous cyst walls are 

 characteristic of Chrysomonadida, and walls composed largely of sand 

 grains are produced in Difflugia (203). Many of the thick-walled cysts 

 show spines, ridges, or other surface markings. A compound cyst wall 

 (Fig. 2. 16, C), composed of two or more membranes, is not uncominon. 



