Branchiobdellae Hirudinea "Oligochaeta" (paraphyletic) and see text The Clitellata are a class of annelid worms, characterized by having a clitellum – the 'collar' that forms a reproductive cocoon during part of their life cycles.

Clitellate annelids are segmented worms characterised by the clitellum or girdle which is located near the head end of mature individuals.

Afterwards it secretes material that forms a cocoon that encircles the animal's body and encloses the eggs and sperm.

[2][3] Earthworms and their kin, in the subclass Oligochaeta, lack eyes but have photoreceptor cells in the skin, especially in the dorsal portion of the anterior end.

They also lack parapodia and appendages on the prostomium, the body and the periproct (terminal segment on which the anus is located).

The body wall includes strong transverse, longitudinal and diagonal muscles which give the animal great flexibility and extensibility.

The subclass Oligochaeta, which includes the earthworms as the largest members of the group, mostly live on land, burrowing in damp soil.

The marine species are mostly tiny and live in the interstices between sand grains, from the intertidal zone to the deep sea.

[6] According to modern phylogenetic analyses, the Clitellata are considered to be a monophyletic clade embedded deep in the polychaetes.