This one started out from a conversation I had with  himeric over how skeletons would be able to eat or pee (they can't, and don't need to), so I came up with this explaination:

How an "undead skeleton" works: (in general)

1. The necromancer casts a spell onto the skeleton, turning it into a marionette puppet.

2. The skeleton has "ghost organs". Ghost, or spirit organs is a spell that will bring a skeleton [back] to life by creating "ghost" organs and muscles allowing for independent movement and some level of autonomy without the constaint direct control of the necromancer. These organs are invisible to all except to those who can see ghosts (like Yorumi here). They do not appear as defined organs, but as chakra flames. Ghost organs also appear if the skeleton is still posessed by the spirit of the [cursed] deceased, in which case, they may still retain some traces of their former selves.

This is only my theory on it of course. But no matter what you believe, it's going to be a long Walpurgis Night for Yorumi.