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Abstract
The main life forms among the Oligochaeta s.l. (= Clitellata) and the related polychaetes are: aquatic (freshwater and marine) sediment-dwellers, inhabitants of the macrovegetation, large and small soil-dwellers, and carnivores. The vegetation-dwellers (Naididae, Pristinidae and Opistocystidae) reproduce mostly in an asexual way; some of them have an ability to swim and posses eyes. A convergent group to the naidid oligochaetes is the aphanoneuran genus Aeolosoma. The smaller Enchytraeidae, and the larger “earthworms” (= Megadrili) Crassiclitellata and Moniligastridae, live in the terrestrial soil. Some Enchytraeidae and Crassiclitellata are secondarily aquatic while some (generally aquatic) tubificids can facultatively live in the soil. Carnivory (as parasitism, commensalism or predation) has been developed in separate genera of several families. A large clade, including the Hirudinea, the Acanthobdellidae and the Branchiobdellidae, is highly adapted for carnivory (suckers, jaws, loss of chaetae, etc.). Two evolutionary trends are evident in different clades: reduction in chaetal number from indefinite to two per bundle, or to complete loss of chaetae, and reduction of the upper tooth in the originally bifid sigmoid chaetae. External gills have appeared at least in four independent cases. There exist many convergencies in the mode of life and morphology of separate Oligochaeta and related “oligochaetoid” polychaetes.