They had such beautiful markings and were really quite big, but I would prefer not to have them living in amongst my lettuces, so I moved them out of harms way to the nettle patch. I believe they're green cellar slugs.
Instead of a shell, they have a leathery patch called a mantle shield. The hole in this shield is called the pneumostome and acts as the slug's breathing pore or nostril. Like most slugs, they have two pairs of feelers on their head. One pair detect light, while the lower pair are used for smell or possibly taste.
|The pneumostome on the right hand side of the mantle|