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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.
any of a group of wormlike marine invertebrates closely related to the chordates and sometimes considered to constitute a phylum, Hemichordata. The term Hemichordata-from the Greek hemi, meaning "half," and chorde, meaning "string," thus, "half-chordate"-was first proposed because the buccal diverticulum, a tubular outgrowth from the mouth cavity forward into the proboscis, or "snout," resembled a rudimentary notochord-the dorsal, or back-side, supporting axis of the more primitive vertebrates. This theory has since been rejected, however, because it has been determined that the diverticulum bears little resemblance in origin and function to the vertebrate notochord. Although the hemichordates are sometimes given phylum rank, some authorities place the group at various other taxonomic levels with the other so-called protochordates.