The Echiura or spoon worms, are a group of marine worms, most of which live in burrows in soft areas. This annelid-like animal group was once considered as a separate phylum because of the absence of segmentation. Although recent molecular analyses have placed it within the annelids. This grouping agrees with the presence and absence of marked sexual dimorphism involving dwarf males and the paired/non-paired configuration of the gonoducts (genital sacs). Furthermore, the data supports the sister group relationship of Echiuridae and Urechidae. These two families share the character of having anal chaetae rings around the posterior trunk as a synapomorphy.

Goto, Ryutaro, Tomoko Okamoto, Hiroshi Ishikawa, Yoichi Hamamura, and Makoto Kato. "Molecular Phylogeny of Echiuran Worms (Phylum: Annelida) Reveals Evolutionary Pattern of Feeding Mode and Sexual Dimorphism." PLOS ONE. Public Library of Science, n.d. Web. 20 June 2017.