Arctonoe vittata, Polychaete worm, found under Black Giant Keyhole Limpet.
The Crane, Palos Verdes, California
ID and comments thanks to Leslie Harris:
a very well known worm because of its association with limpets & other animals. It can live on its own but prefers to be with a host. They are kleptos, stealing bits of food from the host but that a good investment for the host because the worm will defend it from predators, especially sea stars. Scaleworms have 2 big jaws & this guy will immediately come out & start biting at a star's tube feet until the star moves away.