Glyceridae

Gray's Reef National Marine Sanctuary, about 17 miles off Sapelo Island, Georgia, is home to a variety of strange creatures. The Glyceridae, or bloodworms, are ferocious epi- and infaunal polychaetes that prey upon small invertebrates. They are errant burrowers that build galleries of interconnected tubes to aid in catching their prey.
It's a unique animal... or a small monster?

Glyceridae, the small sea monster
Thanks to source : livescience