After Edmonds (2000)
Members of the class Phascolosomatidea have tentacles that are not peripheral, but are arranged so as to form an arc enclosing the dorsal nuchal organ. Introvert hooks are recurved, usually with internal structure or markings and generally are arranged in rings; introvert hooks are absent in species of Antillesoma. The spindle muscle is attached posteriorly. Two orders are included: Phascolosomatiformes and Aspidosiphoniformes.
Edmonds, S.J. 2000. Phylum Sipuncula. pp. 375-400 in Beesley, P.L., Ross, G.J.B. & Glasby, C.J. (eds). Polychaetes & Allies: The Southern Synthesis. Fauna of Australia Vol. 4A Polychaeta, Myzostomida, Pogonophora, Echiura, Sipuncula. Melbourne : CSIRO Publishing Vol. 4 Part A xii 1-465 pp.