[ref. ID; 4386 (Dimas Fernandez-Galiano and Pilar Calvo, 1992)]

The traditional group of hypotrich ciliates (Order Hypotrichida), from Levine et al.'s classification (1980) has lately been the object of several research papers expressing new ideas on the systematics and taxonomic position of this group (Corliss 1979, Fleury et al. 1985, Lynn & Corliss 1991, Lynn & Small 1990, Puytorac et al. 1976, Small & Lynn 1981, Small & Lynn 1985, Tuffrau 1987). Small & Lynn (1981) begin with the concept of "structural conservatism", first exposed Lynn (1976) defending the thesis that "the conservation of structure through time is inversely related to the level of biological organization". These ideas, together with several observations of hypotrich cortical ultrastucture, led those authors to remove the hypotrichs from the Class Spirotrichea and include them in a new class, the Nassophorea. Attention is called here to the fact that the Class Spirotrichea (with the Order Heterotrichida and others) is placed closely to the Class Karyorelictea, which is the most primitive of the ciliate classes. Small & Lynn (1981) also point out the great difference within the hypotrich group; so, the hypotrichs should be split into two groups: Euplotes and other related genera, and hypotrichs sensu stricto, which takes us back to Faure-Fremiet (1961) who was the first to propose division of the Order Hypotrichida (into the suborders Stichotrichina and Sporadotrichina). However, in their paper, Small & Lynn (1981) keep the unit within the hypotrich group. Later, in their monograph on ciliates in the "Illustrated Guide to the Protozoa" (1985). Small & Lynn clearly split the hypotrich group, keeping the euplotids in the Class Nassophorea (subclass Hypotrichia) and removing the rest to the Class Spirotrichea along with heterotrichs and others. To the contrary, French authors who have recently paid attention to the hypotrich classification maintain the hypotrichs as a single group for various reasons. So, Fleury et al. (1985), according to the study of somatic ciliation during morphogenesis, divide the Order Hypotrichida into the suborders Euhypotrichina and Pseudohypotrichina (practically corresponding to the Stichotrichina and Sporadotrichina of Faure-Fremiet (1961)). This position is criticised by Tuffrau (1987), who argues that the new suborder name is inappropriate, instead he proposes a new classification, based on the heterogenic nature of the group Pseudohypotrichina, in an attempt to reconcile recent morphogenetical data with the acquired previously on the infraciliary morphotypes. According to Tuffrau (1987), who emends Jankowski's (1979) characterization of the suborder Euplotina, the Order Hypotrichida is divided into four suborders, keeping the classic Stichotrichina and Sporadotrichina (1961) and adding the Euplotina (1979) and Discocephalina (Wicklow 1982). Finally, in the most recent work on the Ciliophora Lynn & Corliss (1991) present a new phylogenetic tree in which the Euplotidae are again placed with the heterotrichs, oligotrichs, and stichotrichs in the Class Spirotrichea. This is a change from the earlier work (Small & Lynn 1981) in which hypotrichs were transferred into the Class Nassophorea. In our opinion, this last conclusion (Lynn & Corliss 1991) is the correct one. The phylogenetic line from the Suborder Stichotrichina to the Suborder Sporadotrichina seems to be accepted by different authors, following Faure-Fremiet (1961). Therefore, the classification of hypotrichs would change in the following way from that of Tuffrau (1987):

Order Hypotrichida Stein, 1859
Suborder 1: Phacodiniina Small & Lynn, 1985
Suborder 2: Euplotina Jankowski, 1979
Suborder 3: Discocephalina Wicklow, 1982
Suborder 4: Stichotrichina Faure-Fremiet, 1961
Suborder 5: Sporadotrichina Faure-Fremiet, 1961