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.
About 1870 the question arose for discussion whether the somites in front of the mouth are to be considered as derived from the prostomium of a Chaetopod-like ancestor.
In any case the cavity of the prostomium is single, and not formed, as is the cavity of the segments of the body, by paired coelomic chambers.
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.
The body is composed of a large number of segments; the prostomium bears a pair of tentacles; the nervous system consists of a brain and longitudinal ventral nerve cords closely connected with the epidermis (without distinct ganglia), widely separated in Saccocirrus, closely approximated in Protodrilus, fused together in Polygordius; the coelom is well developed, the septa are distinct, and the dorsal and ventral longitudinal mesenteries are complete; the nephridia are simple, and open into the coelom.
The large prostomium is ciliated ventrally.
The segmentation is of the mesoblast to begin with, and appears later behind the mouth, the part anterior to this becoming the prostomium of the adult.
Kleinenberg), in which case there is more reason for favouring the view that would assign an equality between the prostomium and the (in that case) other segments of the body.
The setae are implanted metamerically in accordance with the metamerism of the body, which consists of a prostomium followed by a number of segments.
Nerve cord lies in coelom; brain in first segment or prostomium in many forms. Clitellum generally only two or three segments and more anterior in position than in Terricolae.