Changes in the form of the buccal cavity and oesophagus may accompany these moults, but the sexual organs do not become fully differentiated until after the third moult. In many forms the cuticle of the second moult is not immediately shed, but serves as a loose protective sheath until the larva finds its proper habitat. This is especially the case with the "infective" larvae of many parasitic forms, which do not shed the second cuticle until they find their way into the host At this "ensheathed" stage larval Rhabditidae and Mermithidae, which are partly parasitic, and the Anguillulinidae (Tylenchidae), many of which attack plants.
The life-history of the Ascaroidea is usually direct, though some of the Ascaridae require an intermediate host. The infec tive larvae of Ascaris and some other genera have a complex migration within the body of the final host, travelling by way of the blood-stream to the lungs, whence they return by the trachea to the mouth, before finally settling in the intestine. In the Oxyuridae—e.g., the common "threadworm" or "pinworm" (Enterobius vermicularis) of man—the eggs, as soon as laid, commonly contain infective larvae, rapid infection of new hosts, or constant reinfection of the same host, thus being possible. In some of the Rhabditidae (including the human parasite, Strongy loides stercoralis) a free-living bisexual generation alternates with a parasitic generation consisting of "female" forms only. In the Mermithidae the sexual forms are free-living, the larval stages parasitic, chiefly in insects.
II. Strongyloidea. Parasitic forms in which the male is pro vided with a caudal "bursa" supported by a definite system of "rays." The more important families are the Strongylidae, Ancylostomidae (hookworms), Metastrongylidae and Tricho strongylidae.
The life-history is usually direct, the infective larvae usually being taken in by the mouth, but in some cases—e.g., the hook
worms—being able to penetrate the skin. Migration within the host's body is frequent. In the case of the hookworms it follows a similar course to that of Ascaris. The adults of the Meta strongylidae inhabit chiefly the air-passages and blood-vessels of mammals, and are often responsible for bronchial and other dis orders. The adults of the other families inhabit mainly the alimentary canal, with the exception of one small group which includes the "gape-worms" (Syngamus) of birds and mammals.
III. Filarioidea. Parasitic forms with paired lateral lips, or with or without secondary lip-like structures. The principal families are the Filariidae, Spiruridae, Camallanidae, Cucullanidae and Gnathostomidae.
The life-history is probably indirect in all cases. The adults of the Filariidae occur mainly in the connective tissue or blood vessels of vertebrates, while the development of their larval stages takes place partly in mosquitoes or other blood-sucking insects, by which infection is transmitted. The Guinea-worm (Dracun culus) is transmitted by small water-fleas (Cyclops) contained in drinking-water. Among the other families the larvae usually become encapsuled in some intermediate host liable to be ingested by the final host, whose alimentary canal is the habitat of the adults.
IV. Dioctophymoidea. Containing a single family, Dioctophy midae, with three genera of parasitic forms chiefly characterized by the presence, in the male, of a caudal bursa-like structure without rays. To this group be longs Dioctophyme, the largest known nematode, a parasite of the urinary system of various mammals.
V. Trichinelloidea. Containing only the family Trichinellidae, parasitic forms in which the oesophagus is a cuticular tube em bedded in a single chain of cells.
To this group belong Trichinella, the cause of "trichinosis," and the "whipworms" (Trichuris).
The life-history is usually direct, but the adults of Trichinella, living in the intestine, give rise to larvae which become encapsuled in the muscles of the same host, and transmission to a new host depends upon the ingestion of the flesh containing them.