A brief history of acoel classification
The name Acoela was introduced by Uljanin (1870) to denote free-living flatworms (“turbellarians”) without an intestinal lumen.
An early, pre-cladistic classification of Acoela in the two families Proporidae (acoels with one gonopore) and Convolutidae (acoels with two gonopores) was proposed by von Graff (1905; 1911), who also elevated Acoela to the rank of subclass. The diagnoses of Convolutidae and Proporidae were modified by Luther (1912) to instead include acoels possessing a copulatory bursa (a secondary female organ for storage of allosperm) in Convolutidae and those lacking a copulatory bursa in Proporidae.
Einar Westblad studied the Swedish acoel fauna in a series of papers where he gave detailed accounts of their anatomy and proposed a comprehensive classification of Acoela, again working in a pre-cladistic framework (Westblad 1940, 1942, 1945, 1946, 1948). Westblad’s system was based on the characters that he perceived as the most important for the systematization of Acoela, namely the histology of the gonads, presence or absence of female copulatory organs and the position and anatomy of the male copulatory organ. Among these characters, Westblad identified several primitive states: follicular gonads, mixed male and female gonads, posterior male gonopore, and lack of copulatory organs. Other primitive states proposed by Westblad (1948) were an epithelial nervous system, and a ventral mouth without a pharynx. The families Diopisthoporidae, 1940, Haploposthiidae, Hallangidae, 1946, and Otocelididae, 1948 were introduced by Westblad in addition to Proporidae and Convolutidae. Papi (1957) introduced the family Hofsteniidae for Acoela with an exceptionally muscular pharynx. Paratomellidae was introduced by Dörjes (1966).
A major transformation of acoel taxonomy was carried out by Dörjes (1968), who named a large number of new species and introduced the families Anaperidae (antrum masculinum with more than one sclerotized prostatoid organ), Antigonariidae (monotypic family with a hermaphroditic mixed gonad), Childiidae (conical penis stylet), Mecynostomidae (spherical posterosubterminal male copulatory organ), Nadinidae (with large, partially ciliated pharynx, monotypic) and Solenofilomorphidae (slender acoels with pharynx and posterior ovary) as well as numerous new genera. None of these were based on explicit phylogenetic hypotheses.
Development after the publication of Dörjes 1968 monograph has been incremental. The monotypic family Taurididae was proposed by Kostenko (Kostenko 1989) based on a peculiar pattern of the male copulatory apparatus characterized by consecutive connection of the vesicula seminalis, vesicula granulorum and ciliated antrum. Kostenko and Mamkaev (1990) introduced the Sagittiferidae (saccate male antrum, epidermal extrusomes-sagittocysts). The configuration of the body wall musculature was emphasized as phylogenetically informative by Hooge and Tyler (Hooge 2001; Hooge 2003; Hooge and Tyler 2005) who introduced four new families: Actinoposthiidae, Hooge 2001, to which species with “normal” body wall musculature, i.e. circular muscles in the outer layer, were transferred from Childiidae; Dakuidae, Hooge 2003, a monotypic family with muscular seminal vesicle surrounding a bulbous penis; Polycanthiidae, Hooge 2003 again a monotypic family with multilayered muscular seminal vesicle and no penis; and finally Isodiametridae, Hooge and Tyler 2005 a large family of unpigmented acoels with an isodiametric penis invaginated into the seminal vesicle.