Sausage casing
Cleaned animal intestine, usually from pig or sheep, used to hold sausage filling.
Natural casing is available from butchers on request, sometimes at Turkish grocers, and reliably by mail order. It is usually sold salted, is soaked and rinsed through before filling, and keeps for a long time in the fridge.
Substitute:
Edible collagen casing works for fresh sausages, but it stretches less and splits more easily when boiled. Without any casing, the filling can be rolled tightly in cling film and poached, though it will have no skin and no bite.