Oligochaetes (earthworms, sludge worms, etc.)
Oligochaetes comprise, together with leeches and leech-like worms, the group Clitellata within Annelida (the segmented worms). About two thirds of the almost 4,000 oligochaete species known on the Earth are large forms - we call them earthworms. The earthworms vary in length between about 2 cm and 4 m. Most of them live in soil, some in fresh water, and a single species is common in seashores around the tropical zone of the Earth.
A generalized oligochaete worm
The remaining oligochaetes are more or less microscopical, very thin and from about 1 mm to a few cm long. These smaller species largely live in water (either fresh or salt), only one group (family Enchytraeidae) is living predominantly on land.The group Clitellata, to which the oligochaetes belong, is characterized by that sexually mature individuals bear a delineated, glandular, 'girdle' (clitellum) around one part of the body. In other ways the oligochaetes resemble some of their close relatives among the annelids, the bristle worms (Polychaeta). However, they never possess any paired appendages on their head, and generally they do not have as many bristles, as is typical of the latter.
Tubificoides vestibulatus Erséus & Bonomi, 1987, an oligochaete from the Mediterranean Sea.
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