The Annelida (from Latin "annellus", meaning little ring) are a large animal phylum comprising segmented worms, most familiar as the earthworm. They are characterised by a combination of features, the most obvious of which is a long body composed of nearly identical segments containing a standard set of organs. There are more than 17,000 modern species living in marine, freshwater and moist terrestrial environments, including the ragworms and leeches. Traditional taxonomy divided the annelids into three major groups: the mainly marine polychaetes; oligochaetes, which include earthworms; and Hirudinea or leech-like species, many of which are parasitic. However, this taxonomy has been substantially revised over the last decade on the basis of molecular phylogenetic analyses. For example, the tiny archiannelids have been found to belong within the polychaetes. Enigmatic worms with their reduced body form, such as myzostomids, are still controversial in terms of phylogeny but may now be classified as annelids.

In terms of convergences, annelids are instructive in a number of instances. Most notable are luminescence, moulting and the independent evolution of both compound eyes (especially in sabellids) and camera eyes (in alciopids). Within the group, there are also some interesting examples of convergent evolution. The polychaete Hrabiella, which has unusually adopted a terrestrial lifestyle, shows many oligochaete-like features. Initially considered a convergence due to similar selection pressure, some more recent work has suggested that this genus could actually belong to a sister-group of oligochaetes. Another very remarkable convergence is the evolution in some oligochaetes of love darts, very similar to the more familiar examples in snails and slugs, which are involved in the injection of allohormones probably manipulating sperm competition. A number of annelids build calcareous tubes, of which the small spiral-like forms referred to as spirobids are well known. The spirobid morph, however, is most likely convergent, and it is far from clear that a number of fossil examples are actually the result of tube-building by annelids, let alone polychaetes.