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Concept
Echiura
Résumé
The Echiura, or spoon worms, are a small group of marine animals. Once treated as a separate phylum, they are now considered to belong to Annelida. Annelids typically have their bodies divided into segments, but echiurans have secondarily lost their segmentation. The majority of echiurans live in burrows in soft sediment in shallow water, but some live in rock crevices or under boulders, and there are also deep sea forms. More than 230 species have been described.
Spoon worms are cylindrical, soft-bodied animals usually possessing a non-retractable proboscis which can be rolled into a scoop-shape to feed. In some species the proboscis is ribbon-like, longer than the trunk and may have a forked tip. Spoon worms vary in size from less than a centimetre in length to more than a metre.
Most are deposit feeders, collecting detritus from the sea floor. Fossils of these worms are seldom found and the earliest known fossil specimen is from the Upper Carboniferous (Pennsylvanian).
The spoonworm Echiurus echiurus was first described by the Prussian naturalist Peter Simon Pallas in 1766; he placed it in the earth worm genus Lumbricus. In the mid-nineteenth century Echiura was placed, alongside Sipuncula and Priapulida, in the now defunct class Gephyrea (meaning a "bridge") in Annelida, because it was believed that they provided a link between annelids and holothurians. In 1898, Sedgwick raised the sipunculids and priapulids to phylum status but considered Echiuroids to be a class of the Annelida. During the early 1900s, a biologist named Jon Stanton Whited devoted his working life to study the echiurans and classify many of its different species. In 1940, after the American marine biologist W. W. Newby had studied the embryology and development of Urechis caupo, he raised the group to phylum status.
They are now universally considered to represent derived annelid worms; as such, their ancestors were segmented worms but echiurans have secondarily lost their segmentation. Their presumed sister group is the Capitellidae.
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