Hemichordata (phylum)

Scope note: Small phylum that includes acorn worms and pterobranchs, characterized as wormlike marine invertebrates closely related to the chordates. While they are typically given phylum rank, some sources place the group at various other taxonomic levels with the other so-called protochordates. Hemichordates are so named for the Greek hemi, meaning "half," and chorde, meaning "string," thus, "half-chordate," which was first proposed because the buccal diverticulum, a tubular outgrowth from the mouth cavity forward into the proboscis, resembled a rudimentary notochord. 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. Along with the proboscis, the body comprises a short collar, and a long, wormlike trunk.

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