The echiurans were included in the Annelida until recently, and they are still considered close relatives of the annelids. The body of an echiuran lacks annelid-type segmentation, but the free-swimming larval stages of echiurans and polychaetes are very similar. Echiurans have an extensible proboscis and a set of small hooks at the posterior end; hence the Latin name of the phylum, "spine-tails." In English, echiurans are referred to as "spoon worms" (when referred to at all).
Although there are only about 10 species of echiurans known today, they are quite common in some marine environments. Urechis caupo, the "innkeeper worm" pictured here, is common in some mudflats of the Pacific coast of California. Normally it inhabits a U-shaped burrow; it is shown here in a glass tube, in a laboratory experiment on feeding. The mucus net which it creates with its proboscis is just visible; it filters water through its burrow and traps planktonic organisms. This is an unusual mode of feeding for echiurans; most use their proboscises to move sedimentary detritus to their mouths. Urechis is known as the "innkeeper worm" because a number of marine organisms, including small crabs, polychaete worms, and fish, live as commensals inside the echiuran's burrow.
U-shaped trace fossils are known in the fossil record from Cambrian times; some of them may have been made by echiurans, but a number of other organisms make very similar burrows. Body fossils of echiurans are much rarer, since echiurans have no hard parts; the oldest is Pennsylvanian.
Rudolph Leuckart, the great 19th century zoologist, created a classic set of wall charts in zoology. Click here for his illustration of echiurans and sipunculans (another phylum, probably related to the annelids).
