Saturday morning I saw several really small slugs under the wet rocks in my backyard. This one, barely 5 mm long, was one of them. It is definitely a juvenile, because there are no known species of slugs, at least in the eastern U.S., that don’t grow larger. I suspect this slug was either a Deroceras reticulatum or an Arion intermedius.
Later, while looking at the pictures on the monitor, I noticed the patches of green visible thru the slug’s body (arrows in the picture below). Slugs don’t have green organs. So, I believe what we are seeing is what the slug had eaten, probably some algae growing on the rocks.
Compare the positions of the green patches with the positions of the stomach and the upper part of the rectum in the drawing below. I think it is safe to assume that the lower arrow in the slug picture is pointing at the stomach and the upper arrow is to the upper rectum.
Generalized representation of the digestive system of a pulmonate slug. Figure is from Barker, G.M. 1999. Naturalised terrestrial Stylommatophora. Fauna of New Zealand. No. 38.