Owenia fusiformis is a thin, cylindrical, segmented worm, up to 10 cm long, that lives in a tough flexible tube buried in the sand with its anterior end just protruding from the surface. The tube is composed of sand grains or shell fragments glued together in an overlapping fashion.

Owenia fusiformis, phylogenetic position, sampling location, and embryonic development. a O. fusiformis (blue box) is a member of Palaeoannelida, the sister clade to all remaining annelids. Annelid phylogeny according to [ 11 ]; b, c as adult, O. fusiformis dwells inside a self-built sand tube.