the ground and the young larvae make their way into grasshoppers, in whose bodies they pass most of their larval life. (2) To the second class belong Ankylostoma, Strongylus and many species of Ascaris; the embryo on leaving the egg lives free in water or damp earth, and resembles very closely the free-living genus Rhabditis. After a longer or shorter period it enters the alimentary canal of its proper host with drinking-water, or it bores through the skin and reaches the blood vessels, and is so conveyed through the body, in which it becomes sexually mature. Rhabditis nigrovenosa has a developmental history which is entirely anomalous, passing through two sexual generations which regularly alternate. The worm inhabits the lung of the frog and toad, and is hermaphrodite (Schneider) or parthenogenetic (Leuckart); the embryos hatched from the eggs find their way through the lungs into the alimentary canal and thence to the exterior; in a few days they develop into a sexual larva, called a Rhabditiform larva, in which the sexes are distinct; the eggs remain within the uterus, and the young when hatched break through its walls and live free in the perivisceral cavity of the mother, devouring the organs of the body until only the outer cuticle is left; this eventually breaks and sets free the young, which are without teeth, and have therefore lost the typical Rhabditis form. They live for some time in water or mud, occasion all entering the bodies of water snails, but undergo no change until they reach the lung of a frog, when the cycle begins anew. Although several species belonging to the second class occasionally enter the bodies of water snails and other animals before reaching their definitive host, they undergo no alteration of form in this intermediate host; the case is different, however, in Filaria medinensis and other forms, in which a free larval is followed by a parasitic existence in two distinct hosts, all the changes being accompanied by a metamorphosis. Filaria medinensis—the Guinea worm—is parasitic in the subcutaneous connective tissue of man (occasionally also in the horse). It is chiefly found in the tropical parts of Asia and Africa, but has also been met with in South Carolina and several of the West Indian islands. The adult worm in the female sometimes reaches a length of 6 ft. The males have only recently been discovered. The female is viviparous, and the young, which, unlike the parent, are provided with a long tail, live free in water; it was formerly believed from the frequency with which the legs and feet were attacked by this parasite that the embryo entered the skin directly from the water, but it has been shown by Fedschenko, and confirmed by Manson, Leiper and others, that the larva bores its way into the body of a Cyclops and there undergoes further development. It is probable that the parasite is then transferred to the alimentary canal of man by means of drinking-water, and thence makes its way to the subcutaneous connective tissue.
The Nematoda which are parasitic during their whole life may similarly be divided into two classes—those which undergo their development in a single host, and those which undergo their development in the bodies of two distinct hosts.
(1) In the former class the eggs are extruded with the faeces, and
the young become fully formed within the egg, and when accidentally
swallowed by their host are liberated by the solvent action of the
gastric juice and complete their development. This simple type of
life-history has been experimentally proved by Leuckart to be
Fig. 4.—Trichinella encysted among muscular fibres. (After Leuckart.) characteristic of Trichocephalus affinis, Oxyuris ambigua and other species. (2) The life-history of Ollulanus tricuspis is an example of the second class. Ollulanus tricuspis is found in the adult state in the alimentary canal of the cat; the young worms are hatched in the alimentary canal, and often wander into the body of their host and become encysted in the lungs, liver and other organs; during the encystment the worm degenerates and loses all trace of structure. This wandering appears to be accidental, and to have nothing to do with the further evolution of the animal which takes place in those embryos which are voided with the excrement. Leuckart proved experimentally that these young forms become encysted in the muscles of mice, and the cycle is completed after the mouse is devoured by a cat. The well-known Trichinella spiralis (fig. 4) has a life-history closely resembling that of Ollulanus. The adult worm, which is of extremely minute size, the male being only 1th and the female 1th of an inch in length inhabits the alimentary canal of man and many other carnivorous mammalia; the young bore their way into the tissues and become encysted in the muscles—within the muscle-bundles according to Leuckart, but in the connective tissue between them according to Chatin and others. The co-existence of the asexual encysted form and the sexually mature adult in the same host, exceptionally found in Ollulanus and other Nematodes, is the rule in Trichinella; many of the embryos, however, are extruded with the faeces, and complete the life cycle by reaching the alimentary canal of rats and swine which frequently devour human ordure. Swine become infested with Trichinella in this way and also by eating the dead bodies of rats, and the parasite is conveyed to the body of man along with the flesh of “trichinized” swine.
Importance in Pathology.—Among recent advances having medical import in our knowledge of the Nematodes, the chief are those dealing with the parasites of the blood. F. bancrofti is known to live in the lymphatic glands, and its embryos Microfilaria sanguinis hominis nocturna, passing by the thoracic duct, reach the blood-vessels and circulate in the blood. Manson showed in 1881 that the larvae (Microfilariae) were not at all times present in the blood, but that their appearance had a certain periodicity, and the larvae of F. bancrofti, Microfilaria nocturna swarmed in the blood at night-time and disappeared from the peripheral circulation during the day, hiding away in the large vessels at the base of the lungs and of the heart. Ten years later Manson discovered a second species, Filaria perstans, whose larvae live in the blood. They, however, show no periodicity, and are found continuously both by day and by night; and their larval forms are termed Microfilaria perstans. The adult stages are found in the sub-peritoneal connective tissue. A third form, Microfilaria diurna, is found in the larval stage in blood, but only in the daytime. The adult stage of this form is the Filaria loa found in the subcutaneous tissues of the limbs.
The presence of these parasites seems at times to have little effect on the host, and men in whose system it is calculated there are some 40-50 million larvae have shown no signs of disease. In other cases very serious disorders of the lymphatic system are brought about, of which the most marked is perhaps Elephantiasis. Manson and Bancroft suggested that the second host of the parasite is the mosquito or gnat, and for along time it was thought that they were conveyed to man by the mosquito dying after laying her eggs in water, the larval nematodes escaping from her body and being swallowed by man. It is now held that the parasite enters the blood of man through the piercing mouth-parts at the time of biting. When first sucked up by the insect from an infected man it passes into its stomach, and thence makes its way into the thoracic muscles, and there for some time it grows. Next the larvae make their way into the connective tissue in the pro-thorax, and ultimately bore a channel into the base of the piercing apparatus and come to rest between the hypopharynx and the labium. Usually two are found in this position lying side by side; it would be interesting to know if these are male and female. From their position in the proboscis the larvae can easily enter the blood of man the next time the mosquito bites (Low, Brit. Med. Journ., June 1900; James, ibid., Sept. 1900). Shortly after Low had published his results, Grassi and Noè issued a paper dealing with the larvae of F. immitis, which is spread by means of the mosquito Anopheles (Centrbl. Bakter. I. Abth. xxviii., 1900). The larvae of this parasite develop in the Malpighian tubules of the insect; at a certain stage they cast their cuticle and make their way into the space—part of the haemocoel—found in the labium. During the act of biting the labium is bent back, and as the piercing stylets enter the skin of the sufferer this bending becomes more and more acute. Grassi and Noè think that if the cavity of the labium be full of the larval nematodes this bending will burst the tissue, and through the rent the larvae will escape and make their way into the body of the host. Besides Anopheles, two species of Culex, C. penicillaris and C. pipiens, are also accused of transmitting the larvae. A paper by Noè (Atti Acc. Lincei, ix., 1900) seems to prove beyond doubt that the larvae of F. immitis are transmitted in the manner indicated. The adult worm is chiefly found in the heart of the dog, and usually in the right side, which may be so packed with the worms as seriously to interfere with the circulation (fig. 5). The females produce thousands of larvae, which circulate in the blood, and show a certain periodicity in their appearance, being much more numerous in the blood at night than during the day.
Importance as Pests.—Agriculturists now pay increased attention to the nematodes that destroy their crops. A good example of a fairly typical case is afforded by Heterodera schachtii, which attacks beetroot and causes great loss to the Continental sugar manufacturers. The young larvae, nourished by the yolk