The phylum Annelida includes three main groups: the earthworms, the leeches, and the bristleworms. Annelids are found worldwide, and inhabit terrestrial, freshwater, and marine ecosystems. There are over 15,000 described species.

Characteristics of Annelids

All annelids are segmented. Segments, also called metameres, are structures that occur repeatedly along the body of the animal. Each annelid segment contains units of the circulatory, nervous, and excretory systems. In the earthworms and bristleworms, but not the leeches, segmentation extends to the interior of the body, and includes the coelom, which is partially divided into units by structures called septa.

The annelid body is covered by a moist outer cuticle that is secreted by the epidermis. Both earthworms and bristleworms also possess hairlike setae, composed of chitin, the hard material that also forms the exoskeletons of insects. These are absent in leeches.

Annelids have a true coelom, that is, one that is lined...