This species, Arion ater, comes in a wide variety of colours ranging from pure black, through various shades of brown to this rather fetching pale orange. The lower pair of tentacles are for smelling, the upper pair have eyes at their tips. Notice that the eyes look up, not forwards, suggesting that they might be for detecting predators rather than seeking food. The species identification was made using 'Land Invertebrates' by J.L.Cloudsley-Thompson and J.Sankey (Methuen, 1961). This useful little book (which I picked up at a second hand book shop) mentions that A. atar can only be reliably distinguished from the similar Arion rufus by dissection of their sex organs. However, in a footnote they say that A. rufus is now (ie: 1961) generally regarded as a subspecies of A. atar.
Photo taken beside the lake by Frimley Station, Frimley, UK, on 2006-08-18.