This is a marine ribbon worm. It has a proboscis, which is a prehensile tentacle-like appendage that it uses to seize prey and draw it into its maw. In this case, that’s a human hand.
A cursory Google search suggests that there is little in the way of a marine ribbon worm recipes. This is disappointing. What should I do if I catch a dozen and can’t find a tested means of cooking them? This is obviously an animal idea for human consumption.