Other features of the class Echiura
Trunk contains a single, uninterrupted, large coelomic space that extends into the proboscis.
Lack specialized respiratory organs
Lack distinct gonads. Gametes are produced by the peritoneal lining of the coelom and released into the cavity. They are released by nephridiopores. Most are gonochoristic.
Many, like Urechis caupo, the “fat inn-keeper” produce a large burrow which accommodates numerous commensal creatures.