After Edmonds (2000)
Members of the class Sipunculidea have the tentacles arranged peripherally, encircling the mouth. Introvert hooks, if present, are simple, thorn-like and usually not in rows. The spindle muscle is not attached posteriorly, except in species of Siphonosoma and Siphonomecus. The two orders within Sipunculidea are Sipunculiformes and Golfingiformes.
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.