Compiler and date details
2004 - Megascolecidae updated following Dyne & Jamieson (2004)
30 December 2003 - Adrian M. Pinder (aquatic groups), Tim J. Kingston & Barrie G.M. Jamieson (Megascolicidae)
Oligochaetes are earthworms and their aquatic, marine and freshwater relatives. They are euclitellates with testes anterior to ovaries (the reverse of the arrangement in leeches). The basic genital arrangement may have been four gonadial segments, with two pairs of testes followed by two pairs of ovaries (the octogonadial condition). The acrosome tube is simple, lacking the anterior extension seen in Hirudinea; and the spermatozoal flagellum lacks the asymmetrical marginal fibre flanking the axoneme which occurs in branchiobdellids. Jaws, seen in branchiobdellidans and many leeches, are absent. The coelom is spacious, although somewhat restricted in some lumbriculids. Setae are rarely absent (Achaeta in the Enchytraeidae) whereas they are absent from all higher leeches. The basic number of setae is deduced to be four pairs per segment. The prostomium is simple, never with paired sensory appendages (unlike polychaetes), though rarely (e.g. Pristina in the Naididae) with a single median process. Posterior sucker-like structures (basic to leeches) are very rare (the enchytraeid Aspidodrilus) in contrast with their invariable presence, with restriction of segment numbers in branchiobdellidans and leeches. Like all euclitellates, oligochaetes are hermaphrodite and insemination involves copulation (as in leeches but unlike most polychaetes, in which exceptions with copulation include the clitellate Questa) in which sperm are usually received in spermathecae. Fertilisation is only rarely internal (in Eudrilidae) whereas in leeches it is always internal, by means of ectospermatophores or vaginal insemination). Ectospermatophores are rare in oligochaetes (Criodrilus, Almidae).
In a classification based on a large set of somatic and genital characters subjected to a cladistic analysis, four subclasses are recognised (Jamieson 1988). Detailed definitions of the superfamilies and families are given by Jamieson (1978, 1980b). Although the 1988 classification is more complex than that advanced, without recourse to parsimony cladistics, by Brinkhurst & Jamieson (1971), it agrees in supporting the orders Lumbriculida and Moniligastrida. However, it restricts the Haplotaxida and recognises the order Opisthopora for the remainder of the Haplotaxida (including all the 'earthworms') of the earlier work.
The taxa Microdrili and Megadrili of earlier usage, no longer have currency (in modern terms the Microdrili is a paraphyletic assemblage and some Megadrili are as small as some Microdrili), but the terms 'microdrile' and 'megadrile' will sometimes be used for convenience in this account for small, mainly aquatic and large, mainly terrestrial worms.