The prostomium bears often processes, both dorsal and ventral, which in the Sabellids are split into the circle of branchial plumes, which surround or nearly surround the mouth in those tube-dwelling Annelids.
Among the burrowing and tubicolous forms it is not uncommon for the body to be distinguishable into two or more regions; a "thorax," for example, is sharply marked off from an "abdomen" in the Sabellids.
Benham's division into Phanerocephala in which the prostomium is plain, and Crytocephala in which the prostomium is hidden by the peristomium adopted by Sedgwick, can only be justified by the character used; for the Terebellids, though phanerocephalous, have many of the features of the Sabellids.
These worms are in some respects like the Sabellids (Cryptocephala).
The prostomium has many long filaments which recall the gills of the Sabellids, &c. The nephridia are specialized into two series, as in the last-mentioned worms. (5) Spioniformia (including Chaetopterus, Spio, &c.) and (6) Scoleciformia (Arenicola, Chloraema, Sternaspis) are the remaining groups.