Here is another installment in what is turning into a series of posts on slug development and anatomy.
Last night I dissected a specimen of Arion subfuscus, a European slug that has long been naturalized in the U.S. I cut the mantle along 3 of its edges and turned it inside out. Here is what the inside of the mantle looks like. The top border would be along the right side of a live slug.
Compare the photo with this drawing of the pallial cavity of Arion distinctus (from G. M. Barker, Naturalised terrestrial Stylommatophora [of New Zealand], 1999).
What is not visible in my photograph is the ureter that runs from the kidney alongside the rectum to the penumostome, the breathing hole. Interestingly, the kidney of Arion is shaped like a donut with the heart sitting in the hole in the middle. The membrane to the front of the kidney is the lung. It is vascularized, but the veins are not visible in my picture. The opening of the rectum, like that of the ureter, is within the penumostome.
Next, I separated the kidney from the mantle to which it was loosely attached and pushed it aside. Underneath was a secret chamber full of shiny jewels, or more accurately, granules of calcium carbonate. Those constitute the "shell" of Arion.
Yesterday's post was about how during the development of another slug, Limax maximus, many pieces of calcium carbonate appear within the mantle and eventually fuse together to form the slug's shell. In Arion the individual pieces apparently remain loose and a one-piece shell never forms. We seem to be witnessing various evolutionary steps in the disappearance of shells as slugs descended from snails.