Hemichordates: Hemichordata

Hemichordates (heh-mee-COOR-duhts) are wormlike sea animals that live alone or in colonies. The type that lives alone, called acorn worms because of the shape of their heads, is a few inches (centimeters) to several feet (meters) long. They have a three-part body plan of a snout, a simple collar, and a trunk. Hairlike fibers covering the body are used for movement and for distributing mucus. The type of hemichordates that lives in colonies is only about 0.04 inches (1 millimeter) long. They also have a three-part body plan, but the snout is short and shield-shaped and the collar is complex, in some species having tentacled arms. Colony-forming hemichordates live in a network of tubes built with mucus from each animal's snout. A third type of hemichordates has only one species, and only its larvae have been found. Larvae (LAR-vee) are animals in an early stage that change form before becoming adults.