Figure 1. (a) Resistant cyst of Euglypha, a testaceous soil amoeba. Note two layers of siliceous scales and plugged aperture. (b) Resistant cyst of Acanthamoeba, a soil amoeba facultatively parasitic in humans. Note thick protective wall and stellate appearance of enclosed body. (c and d) Resistant cysts of Entamoeba, a parasite of the human digestive tract: (c) histolytica type, highly pathogenic; (d) coli type, less harmful commensal. (Note: the various cysts are not drawn to the same scale.)

Figure 2. (a) Resting cyst of Ceratium, a dinoflagellate, with its own wall but also protected by the theca of the organism's vegetative stage, which encases it. (b) Protective cyst of Ochromonas, a chrysophyte heterokontic phytoflagellate: note outer spines and plugged emergence pore. (c) Resting cysts (four) of heliozoan ‘actinopod’ amoeba Clathrulina, within the perforated nonsiliceous single-piece test of stalked vegetative organism. (d) Resting cyst of another well-known heliozoan, Actinophrys, with outermost covering of siliceous scales. (e) Well-protected resting (seasonal) zygospore of Volvox, a chlorophytic colonial phytoflagellate. (f) A resting cyst of Cosmarium, a charophytic phytoflagellate. Note the externally projecting spines. (g) A resistant cyst of Giardia, a zooflagellate which is a harmful intestinal parasite of humans. (Note: the zygospore and the various cysts are not drawn to the same scale.)

Figure 3. (a) Resting or protective cyst (empty) of Nyctotherus, a parasitic heterotrichous ciliate whose cyst has a heavy lid over its apical aperture. (b) Cyst of the hypotrichous ciliate Euplotes, emphasizing the ridged or grooved outer cyst wall. (c) Cyst of another hypotrich, Oxytricha, showing its heavily spiked/spined outer wall, one of several cystic membranes or walls. (d) Large cyst of the colpodean genus Bursaria, with an angular inner wall and a smooth outermost wall; also note the aperture or exit pore. (e) Unusually shaped resting cyst of a species of the suctorian genus Podophrya. (f) Resting cyst of the well known peritrich Vorticella. Note numerous ‘warts’ covering the surface of external wall. (g) Phoretic cyst of the apostome Spirophrya, showing the mode of attachment to the carapace of the crustacean host carrying it around. (h) Protective cyst of the haptorian gymnostome (or litostome) Didinium, whose complex triple-walled covering has been studied in detail by electron microscopy. (i, j) Cysts of the much-studied genus Colpoda, a common soil ciliate easily maintained under experimental laboratory conditions: (i) resting cyst, showing heavy cyst walls for protection from unfavourable environmental conditions; (j) reproductive cyst, showing the result of two fissions of an originally encysted organism: note the much thinner wall of this temporary cystic stage. (k) Reproductive cyst of the hymenostome Ichthyophthirius, a common tropical fish parasite. Note the numerous tomites inside, result of a series of very rapid fissions. (l) Reproductive cyst of the hymenostome Ophryoglena (related to Ichthyophthirius), free-living but a tissue-feeder of a scavenger-type. The number of tomites produced within a cyst is multiple but fewer than found in Ichthyophthirius. (Note: the various cysts are not drawn to the same scale.)

Figure 4. (a) Multicellular spore-containing sorocarp (atop stalked sporophore) of Dictyostelium, a mycetozoan cellular slime mould. Spores (not shown) dispersed on rupture of the sorocarp. (b) The huge gametocystic stage in the life cycle of Gregarina, a sporozoan gregarine parasitic in insects. Outside the host's body, it houses developing oocysts, known as spores (each containing eight sporozoites); the latter will eventually be expelled through everted radial tubes or sporoducts (two fully extended ones depicted), eventually to be taken up from soil by fresh hosts. (c) A mature oocyst (outside the host) of the widespread coccidian sporozoan genus Eimeria, whose species are destructively parasitic in practically all vertebrates except humans, simplified to emphasize thick outer walls, with micropyle at apex, and the contained sporocysts, each with pair of sporozoites. See text for further comments. (d) Minute spore of Thelohania, microsporidian protist with fungal affinities, showing particularly the coiled extrusible filament through which the single contained sporoplasm will be transferred to a fresh host (often an insect). (e) Spore of Myxobolus, myxosporidian protist with invertebrate animal affinities, showing a binucleated sporoplasm and two cnidoblasts (polar capsules), one with extruded filament for attachment in a fresh host (often commercially important fish species). (Note: the various structures are not drawn to the same scale.)