What appears to be the bull's-eye at the center of the butthole is where the creature's mouth is located. In order to feed, said mouth is surrounded by a "cloud of mucus" that catches organic detritus known as marine snow. As far as their appearance goes, researchers at the aquarium offered up this explanation:
"... researchers found that although the worms had segmented bodies, one of their middle segments was inflated like a balloon, giving the animals a distinctive gumball shape. All the other segments were compressed up against the front and back of the inflated segment, like a cartoon character whose nose and hind parts have been flattened in an unfortunate accident."
Or "ass-ident," as it were.
Rather than reacting with flustered embarrassment, Dr. Karen Osborn of the Monterey Bay Aquarium Research Institute was positively giddy at finding this seagoing sphincter clenching for survival in the bottom of the bay. Most similar worms confine themselves to the mud of the ocean floor, but researchers believe that the pigbutt worm is the first yet discovered to take the "evolutionary leap" of living freely and proudly in the open water. Shine on, you crazy evolutionary abomination.