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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. [2] 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 species, such as Echiurus, Urechis, and Ikeda, live in burrows in sand and mud; others live in rock and coral crevices. 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.
[edit] External structureThe body of an echiuran is composed of a sausage-shaped, cylindrical trunk and an anterior proboscis. They are usually a drab gray or brown color, but some such as Bonellia viridis, are green, and others are red or rose. A few are transparent. The proboscis is large, flattened projection of the head and cannot be retracted into the trunk. [edit] FeedingSome spoon worms are suspension feeders. In Bonellia edible particles will settle onto the proboscis and a ciliated channel conducts the food to the trunk. Perhaps the most remarkable feeding adaptations among the spoon worms can be seen in Urechis. U. caupo lives in a large, U-shaped burrow and by pulsating its body it drives water through its lair. To feed, it produces a conical mucus net that lines the burrow as water is sucked in at a rate of about 18L per hour. Edible particles are caught on the net, and after some time the worm slowly eats the net and all the edible matter sticking to it.[3] [edit] Reproduction & defensePoorly studied, with the exception of the species Bonellia viridis, remarkable for its extreme sexual dimorphism and possible antibiotic properties of bonellin, the green chemical in its skin. [edit] Notes and references
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