The Cochlea is a double layered tube filled with fluid.
The Cochlear tube is actually divided in half, length wise. If it is visualized as being straight, a large scale model would resemble a long classroom on the second floor, with a classroom of equal size directly beneath it. At the back of the room (which would be the top of the snail shell) is a stairway leading down into the room beneath. The room beneath is empty except that it is filled with fluid.
Fluid also fills the classroom above.
There is, in addition, at the front of the room below, a round window which is covered with a membrane. This serves as a pressure release when sound waves enter the fluid in the room.
At the front of the room on the second floor, is an oval window covered by a membrane through which the third ossicle (the Stapes) punches in its vibrations.
NOTES: Here is more on the cochlea
NOTES: Here is still more information on the Cochlea.
NOTES: Here is more than you really want to know about the cochlea.