Ciliates are immensely diverse and very active creatures. Studying them alive can be difficult because many species are in constant motion. Their classification also depends on many ciliary characters that are only visible after staining dead cells. This guide illustrates only a few species which are more prominent and easily recognized.

A glossary of ciliate terminology can be found here. The abundance of technical terms reflects both the structural complexity of ciliates themselves, as well as the relatively large number of researchers who have worked on them compared to other protist groups.

A checklist of soil ciliates from Singapore has been compiled by Foissner (2008, Soil Organisms 80 (1): 81-97, pdf).

Traditionally they have been divided based on their ciliary characters. These categories, however, do not correspond to the evolutionary lineages as inferred by other morphological data and molecular data.

  • Holotrichs are evenly covered with seemingly identical cilia.
  • Heterotrichs have some cilia differentiated into specialized structures (“polykinetids”) such as cirri and membranelles.
  • Peritrichs only have cilia in a ring that surrounds their “mouth”.
  • Suctorians do not have cilia as adults, but have ciliated larval stages.

In this guide, for simplicity’s sake, I’ll just group them into sessile ciliates that live attached to surfaces, and free-living ciliates. A recent comprehensive review of ciliate taxonomy is D Lynn (2008) The ciliated protozoa, 3rd ed. Springer.


Sessile ciliates

Free-living ciliates


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