The genus _Cucullanus_ exists in the adult form in the intestines of fishes, and more rarely of reptiles. _C. elegans_ Zed., which live in fresh-water fish, _e.g._ the perch, is viviparous; after birth the young pass into the water and make their way into the alimentary canal of the small crustacean _Cyclops_, and thence into its body-cavity. Here they undergo two moults, accompanied by certain changes in structure. If this second host be swallowed by a fish the parasites are set free, and develop generative organs. {144}_Ollulanus tricuspis_ Leuck., which in the adult state is found in the cat, chiefly in the intestine but also in the bronchi and other parts, gives rise to larvae which are of enormous size compared with the parent; these leave the body, and if eaten by a mouse encyst in its muscles, and if the mouse be devoured by a cat, they complete their life-cycle by becoming sexually mature. The genus _Syngamus_ infests the trachea and bronchi of birds, more rarely of mammals. The _red-_ or _forked-worm_, _Syngamus trachealis_ Sieb., is common in poultry and game birds, and causes the disease known as gapes, which is especially common in young birds, and often gives rise to extensive loss. The peculiarity of this genus is that the male is permanently attached to the female, its genital bursa being so closely adherent to the opening of the oviduct that two specimens cannot be separated without tearing the tissues. The ova are not laid, but escape from the body with fully-formed embryos in them, by the decay or rupture of their parent's body. They hatch in damp earth or water in from one to six weeks according to the temperature. When swallowed by a fowl they develop into adults, which reproduce eggs in less than three weeks. No second host is needed, but the embryos remain alive in the alimentary canal of earthworms, and these doubtless to some extent serve to spread the disease. [Illustration: FIG. 70.—_Syngamus trachealis_ Sieb., natural size and magnified four diameters. The small ♂ is permanently attached to the female. (From Warburton.[183])] III. FAMILY TRICHOTRACHELIDAE. This family is characterised by the anterior end of the body being produced into a long whip-like neck. The mouth is small and devoid of papillae. The oesophagus is very long, and it traverses a peculiar strand of cells. Genera: _Trichocephalus_, _Trichina_, _Trichosoma_, and others.