In his 1934 review of British land mollusks, Boycott discussed their diets and noted that the "partly digested vegetables of rabbit and sheep dung are much appreciated." So he probably would not have been surprised if he had observed with me these mollusks, a Deroceras reticulatum and a Ventridens ligera, feeding on a pile of what was most likely dog poop on a sidewalk late this afternoon.
The sidewalk where I came upon this spectacle was bordered by a forest on one side and a narrow grass strip on the other. The introduced D. reticulatum is quite common around here. They live in backyards of houses, fields and at the edges of forests, whereas the native V. ligera is primarily a forest dweller. In this instance, both species probably came out of the woods, but whether they chanced upon this particular food item while crossing the sidewalk or were attracted to it by its smell, I don't know and I don't intend to carry out experiments to find out.
Interestingly, Chatfield, in her 1976 review of the diets of European snails and slugs, made no mention of them feeding on feces.
A previous post featured an A. subfuscus eating a dead earthworm.
REVISION made 19 October: The original post identified the slug as an Arion subfuscus. Subsequently, Tim Pearce e-mailed me his opinion that the pictured slug can't be an Arion, because its breathing pore is in the back half of the mantle (in Arion the pore would be in the front half of the mantle). He also suggested that it is probably a Deroceras reticulatum. He was correct. I hadn't paid attention to the location of the pore, but instead relied on the slug's color, which is a shade of brown common in A. subfuscus.
Boycott, A.E. 1934. Habitats of land mollusca in Britain. Journal of Ecology 22:1-38.
Chatfield, J.E., 1976. Studies on food and feeding in some European land molluscs. Journal of Conchology 29:5-20.