The unit of measurement is the one-hundredth part of the length of the worm. The measurements are therefore percentages of the length.
The measurements are taken with the animal viewed in profile; the first is taken at the base of the oesophagus, the second at the nerve-ring, the third at the cardiac constriction, the fourth at the vulva in females and at the middle in males, the fifth at the anus.
Taking everything into consideration, it has seemed advisable in the following systematic account of the Nematoda to abandon the larger groups, and to deal directly with the families. Claus distinguishes seven of these, and the diagnoses given at the head of each are mainly taken from his Grundzüge der Zoologie.[[175]]
I. Family Ascaridae.
Body rather stout. A dorsal and two ventro-lateral lips, bearing papillae. Buccal cavity distinct, seldom provided with chitinous armature. The oesophagus often has two dilatations. The tail of the male is ventrally curved, and usually there are two horny spicules. The Ascaridae are found in the intestines of their respective hosts.
Genera: Ascaris, Heterakis, Oxyuris, Nematoxys, Oxysoma, and many others.
Von Linstow[[176]] enumerates over 250 species of Ascaris, of which it will only be possible to mention here one or two. They are all parasitic in Vertebrata.
A. lumbricoides Linn. is one of the largest known Nematodes ♂ = 4-6 in., ♀ = 10-14 in.; Figs. 66 and 67). It is a common parasite in man, and has been found in the ox. It is now generally recognised as the same parasite which inhabits the pig, and which Dujardin regarded as specifically distinct, and named A. suillae. In the latter host, however, it never attains the dimensions it does in man. It inhabits the upper and middle parts of the small intestine, and has been known to escape into the body-cavity and set up abscesses there, or to make its way into the stomach, and to be voided through the mouth. It is practically cosmopolitan in distribution, and is very common in Japan—Baely found it in twenty-one out of twenty-three post-mortems—and in Tonquin and tropical Africa. Heller[[177]] states that no one is free from these worms in Finland, and they are common wherever there is a plentiful water supply, as in the marshy districts of Holland and Sweden. In Iceland alone they seem absent. When examined alive they give off an irritating vapour which seriously affects some observers, causing catarrhal symptoms, which in Bastian's case lasted six weeks. The usual number found in one host is small, one to six or eight, but cases are on record where many hundreds occurred in one person.
The details of the life-history of this form are not yet completely worked out. The eggs leave the body of the host with the excreta, and formerly it was thought they re-entered the alimentary canal in drinking-water, etc., and there developed into the adult without change of host. This view has been combated by Leuckart, who failed to rear the Nematodes by direct feeding, and it has been noticed that the youngest parasites found in the intestine are already 2 to 3 mm. long. Von Linstow has recently suggested that the larval stages may be hatched out in the body of the millipede Julus guttulatus, whose habits might easily lead it to eat the eggs of the parasite in manured gardens, etc., and which is itself sometimes unconsciously eaten when hidden in fruit or vegetables. This would account for the frequent presence of the parasite in pigs, and also for the fact that in man it is commonest in children who are apt to eat windfalls, and in maniacs and people with perverted tastes.
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.