A phylum of free-living, elongate, worm-like, burrowing marine animals, formerly included in the phylum Hemichordata, in which the body is clearly divided into a proboscis (protosoma), collar (mesosoma), and trunk (metasoma). The stomochord extends into the proboscis. The branchial apparatus is well developed, consisting of a long, double row of pores strengthened by a cuticular skeleton (thought to be homologous with the pharyngeal gill clefts of Chordata). The nervous system is notoneural. There are many living genera. No fossil examples are known.
Subjects: Zoology and Animal Sciences.