Canu in 1892 proposed two groups, Monoporodelphya and Diporodelphya, the copulatory openings of the female being paired in the latter, unpaired in the former.
Canu (1892) distinguishes eight sub-families, Longipediinae, Peltidiinae, Tachidii.nae, Amymoninae, Harpacticinae, Idyinae, Canthocamptinae (for which Canthocampinae should be read), and Nannopinae, adding Stenheliinae (Brady) without distinctive characters for it.
Giesbrecht (1900) considers Canu quite right in grouping together in this single family those parasites of ascidians, simple and compound, which had been previously distributed among families with the more or less significant names Notodelphyidae, Doropygidae, Buproridae, Schizoproctidae, Kossmechtridae, Enterocolidae, Enteropsidae.
(The earlier memoirs of importance are cited in Giesbrecht's Monograph of Naples, 1892); Canu, " Hersiliidae," Bull.
1831), followed by Frech, Canu, de Lapparent and others.