Annelida

Members of the phylum Annelida are segmented worms, such as the earthworm. The 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. Also 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, located in most of the segments, 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.

The phylum Annelida contains several classes of worms, among them the worms found in mud and sand, the familiar earthworms, and the leeches. All annelids reproduce by a sexual method where sperm and egg cells are released into water for fertilization to take place.