肠鳃纲是半索动物门的一个纲。总共约有70多种。大多数种类在泥沙中穴居，或在石块下生活。例如柱头虫（Balanoglossus）。

The acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name. They are closely related to the echinoderms. There are about 90 species of acorn worm in the world, the main species for research being Saccoglossus kowalevskii.