Class Echiuridea Fisher, 1946 is diagnosed by a body wall with well-developed circular and oblique musculature, an anteriorly placed proboscis and the presence of chaetae placed ventrally. All echiurans, except the little known species Sactosoma vitreum, belong to this class.
The class Echiuridea comprises four orders Echiuroinea Bock, 1942; Bonellioinea Saxena, 1983; Xenopneusta Fisher, 1946; and Heteromyota Fisher, 1946. They are distinguished mainly by the arrangements of the body wall musculature, the vascular system and the hind gut.
Edmonds, S.J. 2000. Phylum Echiura. pp. 353-374 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.