Echiurans are closely related to the annelids and were included in the Phylum Annelida. There are about 150 species of Echurians also known as Spoon worms and Innkeeper worms. Unlike the Annelids they have no segmentation of their bodies. They have a worm-like body with a large flattened proboscis projecting forward from the head.

Most spoon worms are suspension feeders eating mainly detritus, pushing their proboscis out of their burrows, with the gutter projecting upwards. Edible particles will then settle onto the proboscis and a channel, lined with tiny beating hairs (cilia) pushes the food into the digestive system. Their digestive system is a highly coiled tube with both mouth and anus.

Their nervous system is made up of a brain near the base of the proboscis and a ventral nerve cord running the length of the body. They do not have a distinct respiratory system, absorbing oxygen through the body wall by diffusion.

Echiuran
Image © Ria Tan Flickr

Echiruans are either male or female. Females are around 8 centimetres (3.1 in) long not counting the proboscis, but males are only 1 to 3 millimetres long, and spend their lives within the uterus of the female. Egga are released into the body cavity of the females where they are fertilized. The fertilized eggs hatch into free-swimming larvae. In some species, the larvae briefly develop a segmented body before transforming into the adult body plan.

The Innkeeper worm gets its name because many marine organisms, including small crabs, polychaete worms, and fish, live as commensals inside the echiuran's U-shaped burrow.

References

http://en.wikipedia.org/wiki/Echiura
http://www.ucmp.berkeley.edu/annelida/echiura.html
http://www.earthlife.net/inverts/echiura.html