Three groups of Protozoa, the ciliates, dinoflagellates and sporozoans have been grouped together as ‘alveolates’ because typical cells in all three groups have a pair of subsurface membranes, forming inflated or flattened alveoli (fluid‐filled cushions), beneath the surface membrane. The close relationship between the groups has been confirmed by molecular sequence analysis.

Keywords: protozoa; ciliates; dinoflagellates; sporozoa

Figure 1.

Diagrams of sections through the surface of members of the three alveolate groups to show the arrangement of membranes. In ciliates (a), rows of cilia (C) and extrusive organelles (trichocysts or mucocysts) (E) emerge between the alveoli which underlie the surface membrane. In dinoflagellates (b), thecal plates may occupy the pellicular alveoli. In sporozoa (c), the two inner membranes are pressed tightly together without any fluid space between, though these inner layers are interrupted at pits called ‘micropores’ (M).

Figure 2.

Examples of the Dinozoa. (a) The protalveolate Oxyrrhis (about 25 μm long). (b) Peridinium with a typical dinoflagellate shape and thick thecal plates (cell about 50 μm long). (c) In Ornithocercus the thecal plates are produced into wide flanges around the flagellar groove and at the posterior (cell about 100 μm long). (d) Prorocentrum (about 50 μm long) lacks an equatorial flagellar groove, but still has flagella which beat in different patterns.

Figure 3.

Examples of Sporozoa. (a) Extracellular gregarine cells like this Gregarina from the gut of a mealworm may reach lengths of 200 μm or more. (b) Coccidians of different genera form characteristic spores, as shown by this Eimeria in which the outer oocyst (35 μm long) contains four sporocysts, each containing two sporozoite cells.

Figure 4.

Examples of the Ciliophora. (a) Stylonychia (about 200 μm long) with a row of membranelles (M) and other cilia formed into bundles as cirri (C). (b) Strombidium (about 30 μm long) with a short band of feeding membranelles (M). (c) Didinium (about 100 μm long) with a ‘proboscis’ (P) for feeding and two bands of swimming cilia. (d) The suctorian Podophrya (about 50 μm in diameter) with tentacles for feeding and an attachment stalk. (e) The soil ciliate Colpoda (about 75 μm long).



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Further Reading

Anderson OR (1987) Comparative Protozoology. Ecology, Physiology, Life History. Berlin: Springer.

Coombs GH, Vickerman K, Sleigh MA and Warren A (eds) (1998) Evolutionary Relationships Among Protozoa. London: Chapman & Hall.

Corliss JO (1994) An interim utilitarian (‘user friendly’) hierarchical classification of the protists. Acta Protozoologica 33: 1–51.

Grell KG (1973) Protozoology, 3rd edn. Berlin: Springer.

Hausmann K, Hülsmann N and Radek R (2003) Protistology, 3rd edn. Stuttgart, Germany: E. Schweizerbartsche Buchhandlung.

Kudo RR (1996) Protozoology, 5th edn. Springfield, IL: Thomas.

Lee JJ, Leedale GF and Bradbury P (eds) (2002) An Illustrated Guide to the Protozoa, 2nd edn (dated 2000). Lawrence, KS: Society of Protozoologists.

Levine ND (1988) The Protozoan Phylum Apicomplexa. Boca Raton, FL: CRC Press.

Puytorac P de, Grain J and Mignot J‐P (1987) Precis de Protistologie. Paris: Boubée.

Sleigh MA (1989) Protozoa and Other Protists. Cambridge: Cambridge University Press.

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Sleigh, Michael A(Jan 2006) Alveolates. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0004348]