I have found lots of good parts but haven't used them yet because the creature is doing a great job as it is. However, today I decided to check out this feature.

I told the necromancer I wanted to work on the creature. Where are all my parts?
Some rotten old ones are in the spares slots, don't want those. How can I throw them out? Will the recently found ones appear? I had them in my inventory but they disappeared when I talked to the necromancer. (I suppose like the recipes disappear after talking to Allen and Wesson?)

In the creature building window are two round slots. What are they for? How can I use them? They're not clickable at the moment.

My creature has a head, obviously, but it says "Class: none, level 23". How can I make it a certain class?