Any of various mostly small, wormlike marine invertebrates once thought to be chordates but now considered more closely related to echinoderms. They may constitute their own phylum, the Hemichordata. The bodies of hemichordates are divided into a feeding organ called a proboscis, a ringlike section called a collar, and a trunk. Hemichordates have a gut, circulatory system, and nervous system and are filter feeders. Acorn worms and graptolites are hemichordates.