The morphology of parasites refers to their physical features. Only those used to identify them are illustrated here. In addition to broad classification of shapes and sizes, specific features such as the heads and tails of nematodes are helpful in identification. Features of the mouth are often used too. Male reproductive parts such as spicules and bursa as well as vulval shape in females are also used as identifying features.
Size is perhaps the most important way of identifying the common sheep parasites.
However, immature worms are smaller than adults. Distributions are shown on the maps. Conditions which favour each parasite are included. Sheep are not usually raised in very arid areas because of lack of water and not in areas of high rainfall because of the risk of disease, including parasitism. Sheep-rearing areas of Australia fall into three major rainfall zones.