Commonly know as Bobbit Worm, the reason why it got this lovely name is due to the fact that the female worm attacks the male penis and feeds it to her young after mating.
Actually this is not true at all and is a fairtail and myth. Go look up the reproductive bioblogy of this worm from a real soruce. The male has no such repro organ
From LeslieH correcting me long ago here on this issue.
Pretty funny, considering that these worms don't have external sex organs, don't have bone plates, don't mate, and don't take care of their young..... it's amazing how widespread that story is on the internet.
There's one big species of Eunice in the Philippines that stands up with it's mouth open & jaws extended waiting for fish to go by. When a fish comes within reach the jaws rapidly snap shut. It's named for the Bobbits because the open jaws look like scissors to some folks.
It's highly unlikely there's an actual bobbit worm in someone's aquarium. On the other hand, eunicids of all sizes are common. The small ones are relatively harmless. Larger ones will eat a wide variety of food & other inverts including corals.
Most polychaetes can regrow their heads if the break is within a certain region of the body. Some polys can regrow an entire body from just a few segments. Eunicids are in the first group