Members of the phylum Annelida include several classes of segmented worms, among them the worms found in mud and sand, the familiar earthworms, and the leeches. Segmented worms all display bilateral symmetry, cephalization, an open digestive system, segmentation, and a body cavity.

The body cavity is a true body cavity called a coelom. This is a fluid-filled space between the innermost cell layer and the outer two cell layers. In this space, the reproductive and digestive organs have evolved into complex structures with complex functions. The digestive system is cushioned in the coelom, so the activities of the digestive system take place without interacting with the inner or outer body walls.

Earthworms and other annelids have numerous segments, each separated from the others by internal partitions. Funnel-shaped excretory units called nephridia are located in most of the segments; they remove water and waste. Needed water is reabsorbed, and waste material passes out of the body through pores in the skin. Each segment has longitudinal and circular muscles that contract, compressing fluid to form a water-based skeleton called a hydrostatic skeleton.

The annelids have a digestive and circulatory system running their entire length. The circulatory system is closed, and blood is circulated by the contraction of several muscular vessels called hearts.

All annelids reproduce sexually.