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 m long). (b) Peridinium with a typical dinoflagellate shape and thick thecal plates (cell about 50 m long). (c) In Ornithocercus the thecal plates are produced into wide flanges around the flagellar groove and at the posterior (cell about 100 m long). (d) Prorocentrum (about 50 m 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 m or more. (b) Coccidians of different genera form characteristic spores, as shown by this Eimeria in which the outer oocyst (35 m long) contains four sporocysts, each containing two sporozoite cells.

Figure 4. Examples of the Ciliophora. (a) Stylonychia (about 200 m long) with a row of membranelles (M) and other cilia formed into bundles as cirri (C). (b) Strombidium (about 30 m long) with a short band of feeding membranelles (M). (c) Didinium (about 100 m long) with a ‘proboscis’ (P) for feeding and two bands of swimming cilia. (d) The suctorian Podophrya (about 50 m in diameter) with tentacles for feeding and an attachment stalk. (e) The soil ciliate Colpoda (about 75 m long).