[4] Spoon worms are cylindrical, soft-bodied animals usually possessing a non-retractable proboscis which can be rolled into a scoop-shape to feed.

[5] In the mid-nineteenth century Echiura was placed, alongside Sipuncula and Priapulida, in the now defunct class Gephyrea (meaning a "bridge") in Annelida, because it was believed that they provided a link between annelids and holothurians.

[7] During the early 1900s, a biologist named Jon Stanton Whited devoted his working life to study the echiurans and classify many of its different species.

In 1940, after the American marine biologist W. W. Newby had studied the embryology and development of Urechis caupo, he raised the group to phylum status.

One of the oldest known unambiguous examples is Coprinoscolex ellogimus from the Mazon Creek fossil beds in Illinois, dating back to the Middle Pennsylvanian period.

This exhibits a proboscis, cigar‐shaped body and convoluted gut, and shows that already at that time, echiurans were unsegmented and were essentially similar to modern forms.

Spoon worms vary in size from the giant Ikeda taenioides, nearly 2 m (7 ft) long with its proboscis extended, to the minute Lissomyema, measuring just 1 cm (0.4 in).

These are lined with numerous minute ciliated funnels that open directly into the body cavity, and are presumed to be excretory organs.

[15] Echiurans do not have a distinct respiratory system, absorbing oxygen through the body wall of both the trunk and proboscis, and through the cloaca in Urechis.

[16] The nervous system consists of a brain near the base of the proboscis, and a ventral nerve cord running the length of the body.

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).

Some are found in deep waters including at abyssal depths; in fact more than half the 70 species in Bonelliidae live below 3,000 m (10,000 ft).

One species, Lissomyema mellita, which lives off the southeastern coast of the US, inhabits the tests (exoskeleton) of dead sand dollars.

[21] 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.

[21] A spoon worm can move about on the surface by extending its proboscis and grasping some object before pulling its body forward.

The front of the trunk is shaped into a wedge and pushed forward, with the two anterior chaetae (hooked chitinous bristles) being driven into the sediment.

[23] Spoon worms are typically detritivores, extending the flexible and mobile proboscis and gathering organic particles that are within reach.

When this is sufficiently clogged up, the spoon worm moves forward along its burrow devouring the net and the trapped particles.

It can lengthen the proboscis dramatically while exploring new areas and periodically reverses its orientation in the burrow so as to use the back entrance to feed.

[25] Other spoon worms live concealed in rock crevices, empty gastropod shells, sand dollar tests and similar places, extending their proboscises into the open water to feed.

At spawning time, the genital sacs contract and the gametes are squeezed into the water column through pores on the worm's ventral surface.

[16] The species Bonellia viridis, also remarkable for the possible antibiotic properties of bonellin, the green chemical in its skin, is unusual for its extreme sexual dimorphism.