Gephyrea, a class of unsegmented marine worms, divided into two distinct sub-groups: (a) the Gephyreans proper, without bristles (G. achaeta)—e.g. Sipunculus (q.v.), Priapulus, Phascolosoma; and (b) the Echiuroids or armed Gephyreans (G. chaetifera)—e.g. Echiurus, Thalassema, Bonellia (q.v.). They live at the bottom of the sea, in sand, mud, or among rocks. While the adults of both sub-groups are not segmented, the larvæ of the Echiuroids are, and on this and other grounds many authorities place them apart from the other Gephyreans and nearer the Annelids.
See Selenka, 'Gephyrea,' Challenger Rep. xiii. (1885); De Mace, Bülow, and Selenka, 'Die Sipunculiden,' in Semper's Reisen im Archipel der Philippinen, part ii. (1884); Rietsch, 'Monograph of Echiuridæ,' Recueil Zool. Suisse, iii. (1886).