Most of the 15 000 known species of annelid worms live in the sea, crawling under rocks on the shore and sea bottom, freely swimming or seeking protection from predators in burrows or tubes. Freshwater and terrestrial annelid radiation has been small except for the earthworms, which are outstandingly successful, and the leeches, which are widespread specialised suctorial predators. Annelids vary in length from 1 mm between sand grains to 3 metres in some Australasian earthworms.

Annelid worms are coelomates with metameric segmentation. The annelid coelom is a large fluid-filled body cavity surrounded by mesoderm, providing an efficient hydrostatic skeleton. Metamerism is the serial repetition of similar parts along the length of an animal, manifested primarily in the separation of the mesoderm into segmental blocks of muscle. In annelids there are usually internal partitions (septa) between the segments. Both the coelom and metamerism improve the effectiveness of muscle contractions, so that active locomotion can be much faster than in acoelomate animals. At the same time these advances demand greater complexity: when an extensive coelom separates the inner and outer tissues, a transport system is required and more elaborate respiratory and excretory organs may need to develop. This complexity then allows more structural differentiation and an increase in size.