There are 150 described species; because of extreme sexual dimorphism these vary in length from only a few millimetres to 50 cm (see left Bonellia viridis where the male is not drawn to scale and is usually 1 - 3 mm long). Echiurans burrow in sediment and feed using a contractile proboscis with a ventral gutter and ciliated food-collecting tip. The proboscis is often flattened and flared towards the end (see B. viridis left and Tatjanella grandis below) and contains the brain. T. grandis below shows the typical feeding posture. Detritus sticks to the mucous-covered surface of the proboscis, then cilia move the particles down a groove or gutter to the mouth. Although it can be contracted and extended the proboscis cannot be drawn into the trunk region. In some species the proboscis is very long, e.g. in Bonellia sp. the proboscis can extend to 2 m, while the trunk is only 8 cm long. Echiuran intestines are long and coiled, see right. They may share a common ancestor in the Annelida, and were once considered Annelids. There is no segmentation or metamerism in extant species, but there is evidence of metamerism (serial segmentation) in the fossil record.

The males are very small and live either on or in the female, usually in groups of about 20.

Bonella viridis above left is found in European waters. It is bright green and has a poisonous skin. Males are just 1 - 3 mm in length and live commensally in the proboscis of the female. Usually around 20 males are found on each female.

Sex determination. When B. viridis is in the egg stage its sex is not yet determined. During the larval stage if the individual comes in contact with a female then the larva will develop into a male, otherwise it will develop into a female. This is because the female proboscis produces a hormone which stimulates the larva to develop into a male.