All Ribbon Worms (Nemertea)

Ribbon worms, the Nemertea, range from a few millimeters to 30 meters when stretched, though most are 20 cm or less, and they may be drab or very brightly colored. They are unsegmented, lack a differentiated head and are most often flattened. What defines the group is an eversible proboscis situated in a cavity called the rhynchocoel. Thickly muscled body walls let a worm change its length and width dramatically and quickly, which is often how it moves. The circulatory system is simple, with a pair of longitudinal vessels joined to lacunae, and oxygen and carbon dioxide diffuse across the body surface. Most species are marine and bottom-dwelling, and around 20 live in fresh water.