Echiuran worms, numbering around 135 (Brusca and Brusca 2003) to 165 (Biseswar 2009) known species, are an exclusively marine/brackish water group of round, unsegmented worms with a short, stout trunk and and a proboscis that can be shortened and lengthened (but not retracted). The proboscis has a broad groove beneath, with the mouth at its base. Many echiurans are quite large. The trunk can range from a few centimeters in length to as long as 40 cm, but the proboscis can reach as much as 1 to 2 meters!
Most echiurans burrow in sand or mud or live in surface detritus or rubble. Some species inhabit rock galleries excavated by other invertebrates. Echiurans occur from the intertidal down to 10,000 meters in depth. They have a large coelom (body cavity) that provides a hydrostatic skeleton against which the body wall muscles operate, allowing them to move and burrow. Most echiurans are detritus feeders, burying their trunk and extending the proboscis over the substrate, with organic particles being trapped by mucus on the proboscis. The mucus and food particles are moved by ciliary action along the ventral proboscis gutter to the mouth. Urechis species, which have a very short proboscis, excavate and reside in U-shaped burrows. They produce a funnel shaped mucus net which is attached to the burrow wall by the proboscis. Water is drawn through the burrow and food particles are captured in the mucus net, which is periodically ingested.
Most echiurans have a simple closed circulatory system (although some echiurans, such as Urechis, lack a circulatory system altogether). The main site of gas exchange is probably the hindgut, through which water is pumped constantly in and out, via the anus, by muscular action. Echiuran sexes are separate and, at least in most cases, fertilization is external. Larvae may drift in the plankton for up to 3 months. Species in the family Bonellidae exhibit extreme sexual dimorphism: females may reach a total length of 2 meters, but males are just a few millimeters long, are reduced in complexity, and live attached to the female. Sex determination seems to depend at least in large part on whether or not a larva settles close to the proboscis of a female; if it does, it rapidly matures into a tiny male.
(Jaccarini et al. 1983; Brusca and Brusca 2003 and references therein)
Although generally similar in appearance to sipunculans, the proboscis of sipunculans can be retracted and the sipunculan anus is positioned anterodorsally (posterior in echiurans) (Brusca and Brusca 2003). The exact phylogenetic placement of the echiurans remains elusive. However, they are clearly closely related to the annelids and recent analyses indicate that they are in fact nested within the phylum Annelida (McHugh 1997; Struck et al. 2007); according to McHugh (1997), members of the former phylum Echiura should now all be referred the family Echiuridae, in which they were originally placed more than a century ago.