(Koren & Danielssen, 1876)
A small species, up to 30 mm in length. The trunk is short, pear-shaped, and typically 2-7 mm in length. The long and slender introvert is about three times the length of the trunk. The tentacular crown is simple, consisting of about 8 small (primary) tentacles arranged in a single circle around the mouth. The introvert lacks hooks, but has minute papillae. The whole surface of the trunk is covered with backwardly directed, scale-like protrusions of irregular shape and varying size. A single nephridiopore is ventrolateral on the anterior trunk whilst the anus is removed to the anterior half of the introvert. The longitudinal muscle layer of the body wall is continuous, not collected into bands.
Internally, one retractor muscle is present, inserted at the posterior end of the trunk; two roots are usually distinguishable (O. squamatum-internal). The oesophagus is attached to the ventral surface of the retractor muscle for most of the latter's length. The proximal part of the intestine makes several longitudinal loops, held by fixing muscles, before coiling in a double spiral supported by a thin spindle muscle. The rectum is long, extending into the introvert when everted to the anteriorly placed anus. A rectal caecum is present. A contractile vessel is not discernible. The right nephridium only is developed; this is attached to the body wall for most of its length.
Little is known of the biology of this distinctive species.
Inhabits mud and sands in depths of 180-1000 m. It is often found in large numbers, along with the closely related Onchnesoma steenstrupi .
The species is widely distributed along the edge of the continental shelf from northern Norway and Iceland to Portugal. In the western Atlantic region it occurs between latitudes 34-24°N. Also reported from eastern Mediterranean in shallow depths (18-56 m).