It's hard to imagine what kind of internal structures in that thing could account for the seemingly, but not quite, "peristalic wave" motions just behind the "head" end of the thing near the beginning of the video, when the "head" has just reached the sand bottom. And yet, later, you see additional versions of these waves behind the "torn" section of the thing, about a third of the way back. It must be, indeed, a strange kind of worm, and a big one, at that. May have been a loser in a predation struggle at some point, and near death, judging by it's morphic irregularities.

Forgot to mention that I'm a little skeptical of the claims that the videographer shot that footage at 70 to 90 foot depth, at 9:00 am. Too much light, too much wave action for that depth and time of day, no evidence of camera lights.

Never thought I'd actually see footage of a Pseudometafilteris marinensis (marine sock puppet).

I saw this in a Japanese animation, except it was wrapped around a woman.

That video makes me want to say "undulate" over and over.