Non-segmented Coelomate Worms

A sipunculan

An echiuran

The nonsegmented coelomate worms includes three groups of broadly similar animals that lack segmentation and are similar in general appearance, though not necessarily closely related one to another.

Scientists believe that each group has features associated with ancestral members of different evolutionary lines.

The three Phylum or groups of nonsegmented coelomate worms include:

• Sipunculans, 320 species of animals that are sometimes called peanut worms because of the resemblance of their contracted bodies to a shelled peanut;
• Echiurans, or spoon worms, which include 140 species of sausage-shaped marine animals that resemble sipunculans in size and general habit; and
• Priapulida, a phylum that has only 16 living and 11 fossil species of cucumber-shaped or worm-like marine animals.

Sipunculans burrow in sand and mud or live in coral or wood excavations, or in old mollusc shells. They are deposit or suspension feeders, obtaining nutrients from the substrate and the water around them.

Echiurans burrow in sand and mud, or live in rock or coral crevices. They are also deposit or suspension feeders living on detritus that is trapped by their mucus and transported to the mouth by special apparatus called the prostomium.

Priapulids are benthic invertebrates that live buried in sand and mud in shallow and deep - but mainly cold - water. They have been recorded in mainly cool waters from Siberia to Antarctica, though tiny species of the genera Tubiluchus are widely distributed, and are found in tropical waters.