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Taxonomische classification
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Koren & Danielssen, 1876

A small species, up to 40 mm in length. The trunk is short and pear-shaped, typically 1-5 mm in length. The introvert is long and slender. The tentacular crown is not developed as such since the oral disk is without tentacles or lobes. The introvert lacks hooks but has minute papillae. The trunk is transversely wrinkled and covered with flat to almost spherical "scales" between which are small papillae; over the posterior trunk the scales are aligned and fused to form longitudinal ridges. The single nephridiopore is ventrolateral on the anterior trunk whilst the anus is removed to the anterior introvert, close to the oral disk. The longitudinal muscle layer of the body wall is continuous, not collected in bands.
Internally, one retractor muscle is present, inserted at the posterior end of the trunk (O. steenstrupi-internal). The oesophagus is not fastened to the retractor muscle as in Onchnesoma squamatum . The middle section of the intestine is coiled in a double spiral whilst the proximal and distal sections, and also the oesophagus and rectum, are gathered into loose loops when the introvert is withdrawn; when everted, these sections traverse the length of the introvert, the anus being close to the mouth. A rectal caecum is present. A contractile vessel is not apparent. The right nephridium only is developed; this is fastened to the body wall for most of its length.
The biology of this species is poorly known.

Inhabits mud and sand in depths of 25-900 m. In deep water it occurs with the closely related Onchnesoma squamatum.

The species is widely distributed in the North Atlantic; in the north eastern region it ranges from northern Norway to the eastern Mediterranean and in the western region from 40-30°N.

Onchnesoma steenstrupi