The Echiura, or spoon worms, are a small group of marine animals. They are often considered to be a group of annelids, although they lack the segmented structure found in other members of that group, and so may also be treated as a separate phylum. However, phylogenetic analyses of DNA sequences place echiurans and pogonophorans within the Annelida. The Echiura fossilise poorly and the earliest known specimen is from the Upper Carboniferous (called the Pennsylvanian in North America). However, U-shaped fossil burrows that could be Echiuran have been found dating back to the Cambrian.
Echiurans are marine worms similar in size and habit to sipunculans. Many genera, such as Echiurus, Urechis, and Ikeda, live in burrows in sand and mud; others live in rock and coral crevices. One species, Thalassema mellita, which lives off the southeastern coast of the US, inhabits the tests (exoskeleton) of dead sand dollars. When the worm is very small, it enters the test and later becomes too large to leave.
The majority of echiurans live in shallow water, but there are also deep sea forms. About 140 species have been described.
Echiurans have a worm-like body with a large flattened proboscis projecting forward from the head. The body is typically drab in colour, but bright red and green species are known. The proboscis is a sheet-like structure, rolled around into a cylindrical tube with an open gutter at the ventral surface. The length of the proboscis varies greatly between species, and in some species is many times longer than the rest of the body. It is probably homologous with the prostomium of other annelids.
Compared with other annelids, echiurans have relatively few setae. In most species, there are just two, located on the underside of the body just behind the proboscis. In others, such as Echiurus, there are also further setae near the posterior end of the animal. Unlike other annelids, adult echiurans have no trace of segmentation.
The digestive system consists of a simple tube running the length of the body, with the anus being at the posterior end. The tube, however, is highly coiled, giving it a considerable length in relation to the size of the animal. A pair of simple or branched diverticula are connected to the rectum. These are lined with numerous minute ciliated funnels that open directly into the body cavity, and are presumed to be excretory organs.
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