: any of a phylum (Echiura synonym Echiuroidea) of marine worms of uncertain taxonomic affinities that have a sensitive but nonretractile proboscis above the mouth
shortened from the stem of New Latin Echiuroidea, former group name, from Echiurus, genus name (from Greek échis "viper" —of uncertain origin— + New Latin -urus "having a tail or rear of such a kind," borrowed from Greek -ouros, derivative of ourá "tail") + -oidea "animals of the nature of (the kind specified)" (from New Latin -oïdēs-oid entry 2 + -ea, neuter plural of -eus-eous) — more at ass entry 2
The taxon Echiuroidea is used as a synonym of the now disused class name Chaetifera by the German zoologist Carl Claus (Lehrbuch der Zoologie, 2. Auflage, Marburg, 1883, p. 346), who perhaps originated it. The genus Echiurus was introduced by the French entomologist Félix Édouard Guérin-Méneville in Iconographie du Règne animal de G. Cuvier, tome 2, Planche des animaux invertébrés, planche 6 (1831), as a renaming of Lumbricus echiurus, introduced by the German naturalist Peter Simon Pallas in Miscellanea zoologica quibus novae imprimis atque obscurae animalium species describuntur (The Hague, 1766), pp. 146-51. The specific epithet echiurus would appear to refer to the chitinous bristles on the worm's posterior end, near the anus. If so, it seems possible that Pallas may have meant echinuris "with a bristled posterior" (see echino-) rather than echiuris. — For more on the etymology of Greek échis "viper" see the note at echinus.