any small marine invertebrate of the class Pterobranchia (phylum Hemichordata). Pterobranchs are found mainly in the Southern Hemisphere, but a few species occur in northern waters. The pterobranch body, like that of the related acorn worm (q.v.), can be divided into three regions: a proboscis (i.e., a tubular sucking organ); a collar with tentacles used to filter food, usually small planktonic plants and animals, from the surrounding water; and a trunk containing a U-shaped gut. There are three genera of pterobranchs. Two of them, Rhabdopleura and Cephalodiscus, live in secreted tubes, organized into a colonial structure called a coenecium. The third genus, Atubaria, lives on hydroids. All three genera are rare. About 21 species have been described.

Most species form colonies of genetically identical individuals (called zooids) that arise from asexual budding. Pterobranchs may be descended from extinct marine animals called graptolites, a group that flourished in the oceans roughly 300 million to 500 million years ago.