- Kingdom: Animalia
- Phylum: Annelida
- Class: Polychaeta
- Subclass: Sedentaria
- Order: Sabellida
- Family: Serpulidae
- Subfamily: Spirorbinae
- Scientific name: Spirorbis spirorbis
- Norwegian: posthornmark
The greenish body of this small tubeworm rarely exceeds a length of 6mm. It lives in an evenly coiled tube spiraling clockwise, in contrast to C. spirillum. The tube diameter is approximately 3 mm. The outer edge of the smooth tube spreads out on the substrate as a distinct peripheral flange. The crown of tentacles are colorless.
It prefers shallow water, below the tidal zone. It is most frequent on algae.
It is widespread in the North-East Atlantic, including the Arctic, the North Sea and the British Isles.