Sipunculoidea

1 :  a group of marine worms of obscure systematic position that are commonly classed as a division of Gephyrea and that lack setae and have the mouth at the end of a retractile introvert similar to a proboscis and usually provided with tentacles and the anus anterior and dorsal

2 in some classifications a :  a group coextensive with Gephyrea b :  a class or other group comprising Sipunculoidea (see sense 1 above) and Priapuloidea