Like the spoon worms, the peanut worms are unsegmented worms. Most species are deposit feeders living in or on sand, mud or gravel. The introvert - the body part including the mouth - can be withdrawn into the body, but is hydraulically extracted when feeding. They have no respiratory or circulatory systems. The water pumped through the body transports all necessary oxygen and nutrients. Most peanut worms reproduce sexually.
The peanut worms have been classified as a separate phylum, but recent genetic studies conclude that they should be regarded as annelids. There approximately 320, all marine species, worldwide, 25 of them have been registered in Norway.