Phascolion Théel, 1875
Generic diagnosis::
Introvert one-half to four times trunk length, with or without hooks. Trunk usually with modified holdfast papillae. Single nephridium present, usually right. Longitudinal muscle of body wall in uniform continuous layer.
Small to medium size (<50mm long), commonly found living in empty mollusk shells.
Other information:
Cutler EB, Cutler NJ (1985) A revision of the genera Phascolion, Théel, and Onchnesoma Koren and Danielssen (Sipuncula). Proceedings of the Biological Society of Washington 98:809-850