The Nematoda which are parasitic during their whole life may similarly be divided into two classes - those which undergo their development in a single host, and those which undergo their development in the bodies of two distinct hosts.
The soil to the depth of many feet, and from it springs the most marvellous tangle of huge trees, shrubs, bushes, underwood, creepers, climbing plants and trailing vines, the whole hung with ferns, mosses, and parasitic growths, and bound together by rattans and huge rope-like trailers.
The parasitic and free-living Nematodes are connected by transitional forms which are free at one stage of their existence and parasitic at another; they may be divided into two classes those that are parasitic in the larval state but free when adult, and those that are free in the larval state but parasitic when adult.
Oxyuris, though chiefly parasitic in the Mammalia, occurs also in reptiles, Amphibia and one or two insects.
Although several species belonging to the second class occasionally enter the bodies of water snails and other animals before reaching their definitive host, they undergo no alteration of form in this intermediate host; the case is different, however, in Filaria medinensis and other forms, in which a free larval is followed by a parasitic existence in two distinct hosts, all the changes being accompanied by a metamorphosis.