The spoon worms (135 species) are cosmopolitan burrowers in marine mud and sand. They range in length from 10 –15 cm in length. Echiurids have a large non-retractable proboscis with a gutter on one side, which they use to comb the substrate for detritus. This food is carried to the mouth by ciliary action. Echiurids appear to be related to annelids and sipunculids. Not all echiurids burrow, some are incarcerated in small natural openings in rocks or shells. The larva is a typical trochophore, resembling a polychaete trochophore.
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The proboscis may be bifurcate as in Bonellia viridis from Naples.
The proboscis may be quite short as in Echiuris vulgaris from St Andrews. It was at St Andrews that Forbes and Goodsir obtained the specimen thrown up on the shore after a severe gale of wind that they described in a joint paper to the Wernerian Natural History Society in Edinburgh in 1841.
Urechis also lives in a tube. It gathers food by drawing in water through a mucus funnel by ciliary action and may pump as much as 29 litres of water a day through its tube by peristaltic pressure.