Hypotrichia

The hypotrichs are a group of ciliated protozoa, common in fresh water, salt water, soil and moss. Hypotrichs possess compound ciliary organelles called "cirri," which are made up of thick tufts of cilia, sparsely distributed on the ventral surface of the cell. The multiple fused cilia which form a cirrus function together as a unit, enabling the organism to crawl along solid substrates such as submerged debris or sediments. Hypotrichs typically possess a large oral aperture, bordered on one side by a wreath or collar of membranelles (small membranous structures made up of fused cilia), forming an "adoral zone of membranelles," or AZM.

== Classification == left|thumb|250 px|Diagram of the oxytrichid hypotrich Stylonychia mytilus In older systems of classification, the term hypotrich comprised all ciliates possessing a relatively flattened body shape, strong cirri restricted to the ventral surface, and a large oral region (peristome) partially surrounded by an "adoral zone of membranelles". From a phylogenetic point of view, this historic grouping--which included both euplotid ciliates (such as Euplotes and Aspidisca), and stichotrichian ciliates (such as Oxytricha and Urostyla)--is paraphyletic. Any natural group, or clade, that contains both Euplotes and Oxytricha would also have to include many morphologically dissimilar spirotrich ciliates, such as the tintinnids and the oligotrichs.