Peanut Worms: Sipuncula

Peanut worms are sea creatures that have bilateral symmetry (bye-LAT-er-uhl SIH-muh-tree). This means that their soft bodies can be divided into similar halves. They are sausagelike and not segmented in any way. Their bodies are gray or brown and are sometimes marked with reddish purple or green. They measure up to 11.8 inches (300 millimeters) long. The front part of the body is called the introvert (IN-treh-vuhrt). The introvert has small hooks that are used to gain traction. At the top of the introvert is the mouth. In some species, a ring of tentacles surrounds the mouth. The introvert can be pulled inside the rest of the body. Muscles pull on the mouth end, turning the introvert inward on itself, in what looks like the opposite of turning a sock inside out. Once withdrawn, the body becomes short, resembling a shelled peanut.

The thicker part of the body is called the trunk and is sometimes covered with small bumps. The soft body wall is supported by two kinds of muscles and a large body cavity filled with fluid. The body cavity is called the coelom (SIGH-lum). Ringlike circular muscles squeeze the body wall and shift fluid forward in the coelom to extend the introvert. Long muscles running the length of the body contract to move the body fluid back and pull the introvert into the trunk. Peanut worms do not have circulatory or respiratory systems. Instead, special cells floating in the fluid carry oxygen and nutrients throughout the body. A kidneylike organ inside the coelom helps filter waste from the body fluid. This waste is expelled from the body through one or two openings opposite the anus (AY-nuhs). The anus is usually located near the upper portion of the body, but in some species it is found on the introvert. The nervous system includes a bundle of nerves inside the tip of the introvert and a nerve cord that runs along the underside of the body.