Anonymous but important

Spadella cephaloptera. Wikimedia commons/Zatelmar

Phylum Chaetognatha, or arrow worms, can be found in marine habitats all over the world, including the Baltic. They are an important part of the plankton fauna, sometimes contributing up to 15% of the biomass. A rich arrow worm fauna is thought to have existed already during the Cambrian period, which took its beginning some 540 million years ago.