- Kingdom: Animalia
- Phylum: Annelida
- Class: Polychaeta
- Subclass: Sedentaria
- Order: Sabellida
- Family: Serpulidae
- Scientific name: Placostegus triqueter
- Norwegian: trekantmark
The calcareous tube is attached to the bottom in full length. The diameter is usually less than 1 cm and the length does not exceed 12 cm. There is a clearly visible keel along the tube. The tentacles are mostly red, brown or blue with light stripes, but other color variations exist.
This tubeworm lives on hard surface, like rocks, shells and ship hulls. Juveniles can be found on algae, but probably fall of when they grow too big. It is rarely encountered deeper than 70 meters.
It is widespread in the North-East Atlantic Ocean, from the Arctic to the Mediterranean.