_G. gigas_ occurs in the adult state in the small intestine of swine; in Europe its first or larval host is believed to be the grubs of _Melolontha vulgaris_ and _Cetonia aurata_, but these beetles are absent from America, though the parasite infests American hogs. Stiles[220] has recently made some experiments which tend to show that in the United States the source of infection is some species of the beetle _Lachnosterna_, and he has succeeded in infecting the grub of _L. arcuata_ by feeding it on the eggs of the parasite; from one larva he took 300 parasites six weeks after feeding it. _L. arcuata_ is, like _M. vulgaris_, phytophagous, but the grubs of both the beetles are fond of frequenting manure heaps and patches of dung, and thus are much exposed to the dangers of infection. _G. echinodiscus_ inhabits the intestine of ant-eaters, having been found in _Myrmecophaga jubata_ and _Cycloturus didactylus_. _G. spira_ lives in the king vulture _Sarcorhampus papa_, and _G. taenioides_ in _Dicholophus cristatus_, a species of Cariama. FAMILY III. NEORHYNCHIDAE.—Sexual maturity is reached in the larval stage. The proboscis-sheath has a single wall. A few giant nuclei only are found in the sub-cuticle and in the lemnisci. The circular muscle layer is very simply developed. The longitudinal muscle-cells are only present in certain places. This family includes two species, _Neorhynchus clavaeceps_ and _N. agilis_, which afford interesting examples of paedogenesis. The sub-cuticle and the lemnisci are dominated by a few giant {185}nuclei, which remain in the embryonic state and do not break up into numerous nuclei as in other forms. The musculature is but little developed and the longitudinal sheath hardly exists. The proboscis-sheath consists of a simple muscular layer, and the short proboscis has few hooks and presents an embryonic appearance. The sexually-mature form lives in the carp, _Cyprinus carpio_; the larval form is found, according to Villot,[221] encysted in the fat bodies of the larva of _Sialis lutaria_, one of the Neuroptera, and in the alimentary canal of the leech _Nephelis octocula_, and successful experiments have been made in infecting some species of the water snail _Limnaea_. _N. agilis_ occurs in _Mugil auratus_ and _M. cephalus_. FAMILY IV. ARHYNCHIDAE.—Short forms with the body divided into three well-marked regions—head, collar, and trunk. The head is pitted, the collar smooth, and the trunk wrinkled, not annulated, in spirit specimens. There is no eversible introvert, and no introvert sheath and no hooks. The sub-cuticle and the lemnisci have a few giant nuclei, and the lemnisci are long and coiled.[222]