As promised in this post, here is a picture of the vestigial shell of the slug Limax maximus.
I cut along the back border of the mantle and then folded it over. The head of the slug is to the right. The shell sits under the mantle in a chamber called the shell sac. Immediately under the shell is the lung. I believe the spongy tissue under the shell and opposite to the lung in the picture is the kidney. The scale is in millimeters.