|
| image = File:Urechiscaupo.jpg | image caption = ''Urechis caupo'' | authority = Newby, 1940〔(Name authority )〕 | subdivision_ranks = Subdivision | subdivision = * Order Bonelliida * * Family Bonelliidae * * Family Thalassematidae * Order Echiurida * * Family Echiuridae * *Family Ikedidae * *Family Urechidae }} The Echiura, or spoon worms, are a small group of marine animals. Once treated as a separate phylum, they are now universally considered to represent derived annelid worms, which have lost their segmentation. The majority of echiurans live in shallow water, but there are also deep sea forms. More than 230 species have been described. The Echiura fossilise poorly and the earliest known specimen is from the Upper Carboniferous (Pennsylvanian). However, U-shaped fossil burrows that could be Echiuran have been found dating back to the Cambrian. ==Distribution and habitat== Echiurans are exclusively marine and the majority of species live in the Atlantic Ocean. They are mostly infaunal, occupying burrows in the seabed, either in the lower intertidal zone or the shallow subtidal (e.g. the genera ''Echiurus'', ''Urechis'', and ''Ikeda''). A few are found in deep waters including at abyssal depths.〔 They often accumulate in sediments with high concentrations of organic matter. 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. In the 1970s, the spoon worm ''Listriolobus pelodes'' was found on the continental shelf off Los Angeles in numbers of up to 1,500 per square metre (11 square feet) near sewage outlets.〔 The burrowing and feeding activities of these worms churned up and aerated the sediment and promoted a balanced ecosystem with a more diverse fauna than would otherwise have existed in this heavily polluted area. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「echiura」の詳細全文を読む スポンサード リンク
|