Description of Echiura

Echiurans, known as Spoonworms, for their anterior prostomium that can roll into a gutter-like shape, are closely related to the annelids. They are found only in marine water, and even in depths of up to 10 000 feet. They are cylindrical in shape and are ventrally ciliated, and their thin collagenous cuticle is unsegmented. Some species contain tiny hooks called chatae along the anterior ventral side used for digging when extended, while others have two circles of chatae around the posterior of their trunk for an anchor. Echiurans move their bodies using peristaltic waves to move themselves through the burrow, and their ciliated prostomium is able to creep, thus their dorso ventral and longitudinal muscles are well developed. These worms have no mechanism of defence as they are soft-bodied, however the pigment in their skin, called bonellin is very toxic to predators, and is intended to paralyse them.
Echiurans have many of the same features of annelids, including a ventral nerve ring a through gut. All Echiurans contain a siphon for transporting any water through their digestive system as well as a hemal system; the only species known to not have a hemal system is Erechis caupo. The sexes are separate, and the gametes are kept in genital sacs, which squeeze them out when the animals are spawning. If any larvae land on a female, a special pigment in their skin, called bonellin (which is toxic to predators) will activate development of male worms, and if the larvae land anywhere else, they will develop into females.
The majority of Echiurans are deposit feeders, living in u-shaped burrows, however the Urechis caupo is a filter feeder. Urechis are able to use the peristaltic movements for pumping food through their body. These unusual animals feed on mostly plankton and are commensal hosts to a few species of tiny clams, polychaete scale worms and the goby fish. Deposit feeders on the other hand stretch their prostomium out of the burrow, which can stretch up to 10 times the length of their trunk, to collect particles from the soil.