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. In the classification system developed by Denis Lynn in collaboration with Eugene B. Small, the subclass Hypotrichia was restricted to euplotids and one small order of marine ciliates called Kiitrichida, while most of the traditional hypotrichs were placed in the subclass Stichotrichia. However, some prominent researchers—notably, the Austrian ciliatologist Helmut Berger—rejected Lynn's nomenclature, and continued to assign non-euplotid "hypotrich" ciliates, such as oxytrichids and urostyloids, to some variant of
Friedrich von Stein's original order Hypotricha. In their revised classification of the phylum Ciliophora, published in 2016, Gao et al. place the "stichotrichs" of Lynn & Small in the synonymous subclass Hypotrichia, and place all euplotid and discocephalid ciliates in the subclass Euplotia.The same high-level taxa are used, without ranks, by Adl et al. in their "Revised Classification of Eukaryotes" of 2012.[https://www.ncbi.nlm.nih.gov/pubmed/23020233 File:Oxytricha hymenostoma - 160x (8999935961).jpg|
Oxytricha sp. File:Oxytricha chlorelligera - 400x (10403483023).jpg|A hypotrich, ventral view File:Stichotricha secunda - 400x - Macronuclei (14994677881).jpg| == References ==