_A. megalocephala_, which is found in the horse, ass, zebra, ox, etc., attains even greater dimensions than the foregoing. The male rarely exceeds 7 inches in length, but the female sometimes reaches 17 inches. They are found in the small intestine of their hosts. Cobbold[178] succeeded in rearing larvae which attained a high degree of organisation when the eggs were placed amongst moist horse-dung, and it seems probable that the larvae pass into the body of their hosts in drinking water; at any rate no intermediate host has yet been found, and Davaine, who fed cows, and Leuckart, who fed horses with the unhatched eggs, both failed to infect the animals they experimented on. _A. mystax_, which lives in cats, dogs, and other Carnivora, has also been found in man. It is provided with fin-like extensions on the side of its head (cf. Fig. 62), and varies much in size in different hosts. When first found in man it received the name of _A. alata_. It becomes sexually mature in about three weeks. One of the most remarkable cycles of development amongst the many curious life-histories met with amongst Nematodes, is that presented by _Rhabdonema_ (_Ascaris_) _nigrovenosum_. The free form of this, formerly known as a distinct species, _Rhabditis nigrovenosa_, lives in the excrement of frogs, and attains sexual maturity in a very short time. The sexes pair, and the fertilised ova give rise to embryos which hatch out within the body of the mother, and then begin to devour her internal organs. After the destruction of the mother, the embryos escape and live in water or slime, and sometimes burrow into water snails, but they undergo no change until swallowed by a frog. Then they make their way into its lungs and grow enormously, attaining a length of almost an inch. This form, parasitic in the frog, is a protandrous hermaphrodite, which first produces spermatozoa and afterwards {141}ova; the latter are fertilised by the spermatozoa, and give rise to rhabditiform embryos, which escape by the alimentary canal and form the free-living sexual stage mentioned above. Thus in the life-history of this form we find an alternation of generation, a sexual free-living form alternating with a hermaphrodite parasitic form. Of the enormous number of other species of the genus, only a very few can be mentioned. _A. transfuga_ Rud. inhabits bears; _A. leptoptera_ Rud., lions; _A. ferox_ H. and Ehrbg., Hyracoidea; _A. depressa_ Rud., vultures; _A. rubicunda_ Schn., pythons; _A. sulcata_ Rud., turtles; _A. mucronata_ Schn., the cod and pike; _A. incurva_ Rud., the sword-fish. [Illustration: FIG. 69.—A male and female _Oxyuris diesingi_ Ham. _in copula_, × 60. _a_, Anus; _b_, oesophagus; _c_, bulb; _d_, testis; _e_, intestine; _f_, ovary. (From Galeb.[179])]