Limits, rank and systematic position of the group is rather vague. For example, Brinkhurst and Jamieson, 1971 treated it as the Suborder Tubificina (in the broad sense, including Tubificidae, Naididae, Phreodrilidae, Opistocystidae, Dorydrilidae and Enchytraeidae) under the Order Haplotaxida. Here we delimit the order only with family Tubificidae and family Naididae, an indisputably monophyletic clade (Erséus, 1990).
Number of species about 1030. 104 species known from fresh waters of NWE.
Description
Predominantly aquatic oligochaetes with plesioporous male duct (male pore lying in segment subsequent to testes) equipped with atrium, and spermathecae in testicular segment. Chaetae in variable number per segment. Usually bifid crotchets present in ventral bundles, while in dorsal bundles there can often occur various modified chaetae. Modified genital chaetae common in many genera.
Distribution
Cosmopolitan.
Ecology
In fresh, brackish and saltwater, both on surface of sediment and inside it, also on aquatic plants. Feeding on detritus, bacteria and unicellular algae; few predatory or parasitic species known.
Reproduction
Mainly sexual, with eggs laid in cocoons of various shape. In some taxa different modes of asexual reproduction prevailing.
Literature
Brinkhurst and Jamieson, 1971: 190; Chekanovskaya, 1981: 174-175; Erséus, 1990: 57-63.