The sipunculan worm Aspidosiphon brocki reproduces asexually by transverse fission into two unequal parts, the smaller part comprising the posterior fifth of the animal. Prior to fission each part regenerates the structures essential to the formation of a new individual. The smaller posterior part (daughter) regenerates an anterior body, including introvert, anterior gut, retractor muscles, and nephridia, whereas the larger anterior part (parent) regenerates only the posterior body wall.