members of the family caudinidae are fairly small, plump sea cucumbers with a thin body wall and no tube feet. they are relatively inactive and live in a "u"-shaped burrow in sand or mud at the bottom of the sea. their tentacles spread out above the sediment to catch food particles and their caudal region may be elongated and also extend to the surface. this may help with gas exchange as they have respiratory trees, a type of water lung, attached to the cloaca.