The finned aluminum part you see in the center of the wheel is actually the brake drum. The rim basically has a giant hole in the middle, with a 8 bolt pattern that fastens around the outside edge of the drum.
Take a look at this site:
8 lug article. There are several pages to it, at the bottom of each page you can go to the next page. It shows what the drums and rims look like when they are separated.