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 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.
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]])
Oxyuris is Meromyarian (see p. [137]), and is characterised by the long capillary tail of the female. It includes another human parasite, O. vermicularis, and it is one which it is difficult to get rid of. The female has the characteristic tail and is about 10 mm. long. The male is smaller. They are found in the caecum and rectum of man, and cause great irritation and sometimes serious functional disturbance. The eggs are laid in immense numbers but perish in water. If whilst still in the egg-shell the larvae are swallowed on fruit or raw vegetables, etc., they are set free in the stomach and small intestine by the action of the digestive secretions. The distribution of this parasite is universal. Besides numerous species that inhabit the alimentary canal of Vertebrates, such as O. ambigua Rud., found in hares and rabbits; O. curvula Rud., in the caecum of horses; O. megatyphlon Rud., in iguanas; several species inhabit the rectum of insects, such as O. blattae, O. diesingi, O. blatticola, found in the cockroach; O. spirotheca, and O. hydrophili in the water beetle Hydrophilus.[[180]]
The genus Nematoxys has the most complex arrangement of muscles of any Meromyarian, and forms a transition to the Polymyarian type. The whole body of both sexes is covered with numerous irregularly scattered papillae. The members of this genus have hitherto been found in snakes, Amphibia, and eels; there are but few species.
Oxysoma is another small genus with but three species, found in the intestines of opossums, frogs, and turtles respectively.
II. Family Strongylidae.
Mouth surrounded by papillae; an armature of teeth or spines often present. The chitinous lining of the intestine projects into the interior as ridges. No oesophageal bulb. The male orifice at the posterior end of the body is surrounded by a bell-shaped bursa.
Genera: Eustrongylus, Strongylus, Dochmius, Sclerostomum, Cucullanus, Syngamus, Pseudalius, Ollulanus, and others.