It's rubbish.
The premise of the movie is inherently absurd; a mad scientist who wants to join three people together to create a single digestive system.
To do this, he kidnaps three people, slashes their tendons so they can't walk and sews the mouths of the second two people of the three to the anus of the person in front.
The idea being that when the first person defecates, this shoots into the person behind's mouth and so on down the line.
The biggest problem with this obviously intended to shock premise is that there is not a set of sutures in the world that would secure a person's mouth to another person's arse, certainly not in conditions like this where one's life was at stake. In reality, the persons in position two and three in the centipede would simply rip themselves free of the sutures and then proceed to beat the mad scientist to a richly deserved death. You would worry about bleeding and the like later; nobody is going to simply stay there on all fours and take a mouthful of runny shit, it's just not realistic. You would get your face away from that person's arse regardless of how painful tearing the sutures was.
It's a bit like understanding why necrosis makes zombie apocalypse stuff impossible; at that point you can no longer suspend your disbelief enough to find it scary. Gross, sure; scary, no.
Stuff like this is made purely for shock value. It has no artistic merit whatsoever, and doesn't even work as a horror film because nothing is left to your imagination. Which is always where the true horror is.