With regard to their occurrence in the different classes of the animal kingdom, they have been most frequently observed in Vertebrates and in Insects. They are comparatively rare in the other large divisions. Many genera are confined to certain hosts: thus _Ascaris_, _Filaria_, _Trichosoma_ occur only in Vertebrates; _Spiroptera_ (with one exception) in Mammals and Birds; _Cucullanus_ in Fishes and Amphibia; _Strongylus_ and _Physaloptera_ in Mammals, Birds, and Reptiles; _Dochmius_, _Pseudalius_, _Trichocephalus_ in Mammals; _Dispharagus_, _Hystrichis_, _Syngamus_ in Birds; _Nematoxys_, _Hedruris_ in Amphibia and Reptiles; _Ichthyonema_ in Fishes; and _Isacis_ and _Mermis_ in Insects. Twenty-two species have been described as parasitic in man, of which perhaps the most dangerous are _Filaria medinensis_, the three varieties of _F. sanguinis hominis_; _Dochmius_ (_Ancylostomum_) _duodenalis_, and _Trichina spiralis_. The Ascaridae, as _Ascaris lumbricoides_ and _Oxyuris vermicularis_, though painful, seldom cause death. The enormous number of parasites harboured by one host is shown by the fact mentioned in Leuckart's _Parasites of Man_, that Nathusius[203] took from a single black stork 24 specimens of _Filaria labiata_ from the lungs, 16 _Syngamus trachealis_ from the trachea, more than 100 _Spiroptera alata_ from the coats of the stomach, besides several hundred Trematodes belonging to several different species (see p. 63). Even this has been surpassed in the case of a young horse, in whose body Krause found 500 _Ascaris megalocephala_, 190 _Oxyuris curvula_, several millions of _Strongylus tetracanthus_, 214 _Sclerostomum armatum_, 287 _Filaria papillosa_, 69 _Taenia perfoliata_, and 6 _Cysticercus_ forms. It is impossible here to enter into a full description of the destruction caused to domesticated animals and crops by the presence of these parasites; full details will be found in books dealing {164}especially with this question, such as Neumann's _Parasites and Parasitic Diseases of Domesticated Animals_. A couple of cases will show how important this matter is to the farmer. Crisp estimates that _Syngamus trachealis_ causes the death of half a million pullets in England every year, and Mégnin states that in a single pheasantry 1200 victims died daily; again, the loss of one-third the crop of beetroot is by no means uncommon when it is infested with _Heterodera schachtii_. These show the practical importance of what at first sight seem quite insignificant animals, and the necessity for the minutest observation, for only when we are fully acquainted with all the details of the life-history of a parasite are we in a position to successfully combat it. SUB-ORDER II. NEMATOMORPHA.