Hypotrichida

(invertebrate zoology) An order of highly specialized protozoans in the subclass Spirotrichia characterized by cirri on the ventral surface and a lack of ciliature on the dorsal surface.

An order of Spirotrichia. These protozoans are commonly considered to represent the pinnacle of specialized development in the evolution of ciliates. Somatic ciliature of the ventral surface has been replaced by cirri; on the dorsal surface it is absent or represented by inconspicuous sensory bristles (see illustration). The adoral zone of membranelles is very prominently developed, and the buccal area may occupy a large part of the ventral surface of the body. The whole body is generally rigid. Hypotrichs occur ubiquitously in fresh- and saltwater habitats. See also Spirotrichia.

Euplotes, an example of a hypotrich.