ABSTRACT
Annelida is a group commonly referred to as segmented worms, and they are found worldwide in most habitats except the most arid or aerial. Earthworms and leeches are the most familiar annelids, but the bulk of the diversity of Annelida lies with polychaetes. These are found in nearly every marine habitat, from intertidal algal mats to the deepest sediments. There are pelagic polychaetes that swim or drift, preying on other plankton, and a few groups occurring in fresh water and moist terrestrial surroundings. Until recently, Annelida was split into three major groups given class rank: Polychaeta (bristleworms), Oligochaeta (earthworms etc.) and Hirudinida (leeches), though this has now been revised and revision is ongoing (see section 1.3). The first annelids were formally named by Linnaeus (1758) and today we estimate that the current number of accepted species level taxa is around 14,000 (9,000 polychaetes, 650 leeches, 150 branchiobdellids and 4,000 oligochaetes), though several thousand more have been named and are considered invalid.
