The Equiurans or spoon worms are the only group belonging to the Annelids that lack body segmentation. Recent studies question the correct classification of Equiurans and argue that they would form per se an independent phylum to Annelids.
Etymologically, the name Equiurans derives from the Greek words echinos (spine) and ourá (tail), as it describes one of their distinctive features, the presence of quetas at the rear end of the body.
The group of Equiurans is made up of a small number of species (230 described), entirely marine, which live among the sand, mud and cracks in the rocks, mostly in shallow waters, although there are some species that are found at great depths. They have a retractable tube (proboscis) with a flattened end, which can extend considerably (the proboscis of the specie Bornellia viridis can reach 2 meters in length) and which they use to capture their food, which is mainly made up of detritus. They also use this retractable tube to move and bury themselves in the marine sediment. In this way, these species stir the bottom, promoting a more diverse ecosystem and therefore more balanced.
In terms of size, they range from species that barely exceed a few millimeters, to species that can reach 50 cm in length. Its body generally has a cylindrical and tubular shape. In some species of Equiurans the sexual dimorphism is very pronounced, with the female being much larger than the male.