Sausage casing
Cleaned natural intestine, usually pork, used to hold sausage filling.
Natural casing can be ordered from a butcher and is often found in Turkish or Eastern European grocers or by mail order, usually salted or in brine, so it needs soaking and rinsing before filling. Pork casing suits thicker sausages, the finer sheep casing thin ones.
Substitute:
Without casing, the filling can be shaped into logs and fried or wrapped in caul fat; caul fat keeps the shape and juiciness well, but neither gives the snap of a cased sausage.