4 (see text) Onchnesoma is one of the two genera that constitute the family Phascolionidae of Phylum Sipuncula, described by Koren and Danielssen established in 1873 as the type species to Onchnesoma steenstrupii.
[1][2][3][4][5] The species of this genus are usually small in size (with a trunk of less than 1 cm in length).
The body wall muscle layers presents continuous.
in length) arranged around the mouth, but those tentacles can be reduced in size or completely absent.
Gallbladder contractile is rarely observed and if present is without villi.