In the 19th century, the echiurans or spoon worms were placed with the sipunculans and priapulids in the Gephyrea, a group thought to form a link between the annelids and the holothurians and generally considered as a class of the Annelida.
The species of the phylum Echiura are almost entirely marine with a few brackish water species.
Coelomate worms; adults unsegmented. The body is plump, with a posterior anus and a proboscis extending forwards from the anterior mouth. The proboscis is long, soft, muscular and often coloured differently from the rest of the body. Its ventral surface forms a shallow ciliated channel along which mucus and detritus are brought to the mouth. The mouth is located at the base of the proboscis and is enclosed by the edges of the channel. On the ventral surface, close behind the mouth, there are usually two hooked chaetae and the pore(s) of one or more nephridia. The nephridia lie in the coelom, attached to the ventral wall and act as genital ducts.
The coelom is surrounded by a muscular body wall. The gut is much longer than the body and lies coiled in the coelom. There is usually a simple closed circulatory system, a ventral nerve cord with a loop into the proboscis, and a pair of excretory organs that open into the gut near the anus. Respiratory exchange takes place through the body wall.
The eggs or sperm do not complete their development in the single ventral gonad but are released in the coelom, where they float freely. When mature, the gametes are spawn into the sea through the nephridia, where fertilization occurs and free-swimming larvae develop. An exception is the genus Bonellia (Bonellia viridis ), in which the fertilization is internal.