This is where you shovel dirt in. At the bottom just above the metal chute, is an opening that can be closed, adjusted, or opened completely by a metal handle (can’t be seen) at the top of the photo. The dirt falls to the riffles and while the bellows is pumped by the wheel the dirt vibrates and hops over the riffles. Can’t see the riffles because they are below the hopper. The heavy stuff stays behind.The screen can be removed if something undesirable gets in the hopper.
I have published a few articles on looking for gold. The links are here: http://john000.hubpages.com/hub/Where-to-Find-Gold-if-You-are-a-Newbie-Gold-Prospecting-the-Best-Places-Using-Gold-Panning
Perhaps a little easier to see how the riffle box is removed.
Seen from the front, you get a better view of the riffle plate or I guess you could call it a box. The black bar in the middle rotates. When it is down it holds the riffle box. When you turn it up, you can pull the riffle box with the heavies at the top of the riffles out and dump the material in a bucket to later pan. Waste material piles up near the wheels on this model. There are a number of variations on the market. There are also directions on the Internet for how to build your own. This wooden model is fairly heavy. Most people want one made of aluminum so it isn’t so heavy to carry.