Reproduction and Life-Cycles 75 



In such cases one of the membranes — the ectocyst of Bursoria (16), the 

 mesocyst of Didiniimi (12), the outer membrane of Volvox (122) — is often 

 thicker and more rigid than the others. This heavy membrane may be 

 continuous like the others, or it may, as in Bursaria (Fig. 2. 17, A), con- 

 tain an "emergence-pore" closed by a thin membrane. The two-layered 

 cyst of Naegleria contains several analogous pores (207). Double or mul- 

 tiple resting cysts are sometimes produced in Colpodidae. The double cyst 

 of Tillina magna shows only one ectocyst, but each of the contained 

 ciliates has its own mesocyst and endocyst (11). 



The protective qualities of cysts vary with the species. Dried cysts of 

 Colpoda ciiciilhis have remained viable for more than five years (69). 

 Cysts of Naegleria gruberi also withstand drying (207). Drying at room 

 temperature prolongs the life of protective cysts of Stylonethes sterkii but 

 kills those of Euplotes taylori (90). Cysts of Didinium nasutum do not 

 survive desiccation although they have remained viable for ten years in 

 sealed containers of hay infusion (10). Cysts of Endamoebidae also do 

 not survive drying. However, cysts of Entamoeba histolytica, kept moist 

 under refrigeration, have remained viable for 46 days (245). Woodruffia 

 metabolica produces two types of resting cysts, a stable one which resists 

 desiccation, and an unstable type which does not (136). Resistance of 

 protective cysts to unfavorable temperatures is sometimes striking. Thor- 

 oughly dried cysts of Colpoda have resisted exposure to dry heat at 100° 

 for three hours (23), and immersion in liquid air for 13.5 hours (230). 



Reproductive cysts are those in which fission, budding, and sometimes 

 gametogenesis and syngamy occur m different species. However, repro- 

 ductive activities are not limited entirely to reproductive cysts, since 

 mitosis occurs in the protective cysts of Giardia and various Endamoe- 

 bidae. The wall of the reproductive cyst, although sometimes compound, 

 is usually thin and has relatively little protective value. Such cysts are 

 known in various dinoflagellates (Fig. 2. 15, E) and in certain free-living 

 and parasitic ciliates. Fission within a cyst is characteristic of Colpoda 

 ciicullus and related species (Fig. 2. 15, F). A similar cyst serves also for 

 attachment of Ichthyophthirius m,ultifiliis to the substratum (172). The 

 gametocyst of gregarines probably should be included in this type. Cysts 

 which are presumably of the reproductive type have been referred to as 

 "feeding cysts" in certain dinoflagellates, because they are formed after 

 the organisms ingest a large amount of food. 



Encystment 



Precystic changes in the organism usually precede secretion of a 

 cyst wall. Material for the membrane sometimes accumulates as globules 

 in the peripheral cytoplasm. Food vacuoles may be eliminated, as in 

 Endamoebidae, and cytoplasmic reserves such as starch or glycogen are 

 often stored in abundance. Since cysts usually approach a spherical form. 



