The axial proboscis is generally a symmetrically developed structure which is axial in that it is a muscular specialization of the anterior region of the alimentary canal is not limited to one particular area (e.g., ventrally). Polychaetes with an axial proboscis, recognized by Dales (1962), comprise, as he seems to have been aware, a polyphyletic group. They belong to all three divisions of the Polychaeta: Scolecida, Spionida and Phyllodocida in the system of Rouse and Fauchald (1997). There are two fundamentally different kinds of axial proboscis and they are probably not homologous. On the results of Rouse and Fauchald (1997) it may be that an axial type of proboscis has evolved three times.

1. An eversible, simple axial proboscis. This condition is found in members of Scolecida and Spionida and may have evolved independently in each group from the simple ventral buccal bulb condition.

2. A muscular axial proboscis is found in all members of Phyllodocida, a very large clade of polychaetes. This form of proboscis is frequently armed on its inner wall with jaws which are exposed when the proboscis is extruded.