1. History of the classification of the family Metastrongylidae is briefly reviewed.
2. The probable origin of the family is discussed. It is believed that the metastrongylids arose, in common with the family Strongylidae, from a primitive group of eustomatous, cloacinin-like, gastro-intestinal strongylines that were symbiotic with an ancestral mammalian group, possibly the pantotheres. One group of these symbiotes established itself in the mammalian respiratory tract and originated the metastrongylid line.
3. The family Trichostrongylidae is regarded as of common origin with the family Ancylostomatidae from eustomatous ancestors. Thus meiostomy in the metastrongylids and trichostrongylids is convergent.
4. The Metastrongylidae consist of three main lines, each characteristic of a placentate order: the Filaroidinae in the Carnivora, the Pseudaliinae in the Cetacea, and the Protostrongylinae in the Artiodactyla. These lines probably have evolved from a common ancestor, symbiotic with a host at the base of the carnivore-cetacean-ariodactyl radiation. Certain forms have become secondarily adapted to symbiosis with members of other placentate orders.
5. Morphological and biological evidence is given for each of the foregoing phylogenetic conclusions.
6. Metastrongylus Molin, 1861, is regarded as a relict genus. With it, in a subfamily Metastrongy-linae, is placed the genus Heterostrongylus Travassos, 1925.
7. Heterostrongylus is represented by a single known species, H. heterostrongylus Travassos, 1925, from an opossum (order Marsupialia). Morphologically it is primitive with cloacinin-like cephalic features. In view of the phylogenetic status of its host it may well represent an ancestral type that has evolved along with the Marsupialia from a common ancestry with the other metastrongylids, probably in the panthotheres. As yet unknown lungworms of marsupials may shed light on this interesting point.
8. Diagnoses are given for the family Metastrongylidae and its subfamilies; the twenty-two component genera are listed under the latter. New synonymies are made: Neometastrongylus Kreis, 1944, a synonym of Neostrongylus Gebauer, 1932; Neometastrongylus buechii Kreis, 1944, of Neostrongylus linearis (Marotel, 1913) Gebauer, 1932; and Otophocaenurus Skriabin, 1942, of Pharurus Leuckart, 1848. Otophocaenurus oserskoi Skriabin, 1942, becomes Pharurus oserskaiaae (Skriabin, 1942, n.emend.) n.comb.; Metathelazia massino becomes M. massinoi (Davtian in Skriabin, 1933, n.emend.) Dougherty, 1943.