Hesionidae are a family of phyllodocid "bristle worms" (class Polychaeta). They are (like almost all polychaetes) marine organisms. Most are found on the continental shelf; Hesiocaeca methanicola is found on methane ice, where it feeds on bacterial biofilms. A characteristic apomorphy of the Hesionidae are the of the anterior segments, which are well-developed cup-like sheaths; the cirri of the subsequent segments insert into the parapodia directly, or with just a vestigial cirrophore.