Familia Metridiidae

Carlgren, 1893

Acontiaria possessing a more or less well developed mesogloeal sphincter muscle and lacking ectodermal longitudinal muscle. Column differentiated into scapus and capitulum, with a parapet and fosse. Column smooth, lacking suckers, tubercles, etc, but inconspicuous cinclides are present on the scapus. Mesenteries not divided into macrocnemes and microcnemes. Nematocysts of acontia b-mastigophores and p-mastigophores, the latter occasionally absent.