Many cisterns are infested with Guinea worm (filaria medinensis, Gm.).
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.
Filaria medinensis - the Guinea worm - is parasitic in the subcutaneous connective tissue of man (occasionally also in the horse).
Ten years later Manson discovered a second species, Filaria perstans, whose larvae live in the blood.
The adult stage of this form is the Filaria loa found in the subcutaneous tissues of the limbs.