[One, Two, Middlesboogie]

(Oh dear, that's three ppl now who have that personal pic. This is going to get v. confusing!)

I think that yes, he does have one. There has to be some kind of outlet for excreta (how does he fart if he has no bum?). Because Muds are bipedal, like us, it is likely that they also have a gluteus maximus, forming the buttocks. As for peeing, I reckon that the urea and faeces of Muds come out from the same outlet, just as they do in birds, from which the Muds are descended.