PTEROBRANCH

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 body of a pterobranch, 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.