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Enteropneusta (acorn worms) 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.
acorn worms See ENTEROPNEUSTA.