Echiura
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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. [1] 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 structure
The 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] Reproduction & defense
Poorly 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
- ^ Struck, TH & et al. (2007-5-27), “Annelid phylogeny and the status of Sipuncula and Echiura”, BMC Evolutionary Biology (BioMed Central) 7 (57), ISSN 1471-2148, <http://www.biomedcentral.com/1471-2148/7/57/abstract>
