Amongst the Nematodes with but a single host a remarkable parasite in wheat has its place. This form, known as Anguillula scandens, inhabits in the adult condition the ears of wheat, in which it lays its eggs. After hatching, the larvæ become encysted, but become free on the death of the plant. They now inhabit moist earth, but eventually make their way into the ears of the young wheat and become sexually mature.
The second group of parasitic Nematodes with two hosts may be divided into two groups, according to whether the larva has a free existence before passing into its first or intermediate host, or is taken into it while still in the egg. In the majority of cases the larval forms live in special connective-tissue capsules, or sometimes free in the tissues of their intermediate hosts; but the adults, as in the cases of other parasitic Nematodes, inhabit the alimentary tract.
The life history of Spiroptera obtusa may be cited as an example of a Nematode with two hosts in which the embryo is transported into its intermediate host while still within the egg. The adult of this form is parasitic in the mouse, and the ova pass out of the alimentary tract with the excreta, and may commonly be found in barns, etc. If one of the ova is now eaten by the meal-worm (larva of Tenebrio), it passes into the body cavity of this worm and undergoes further development. After about five weeks it becomes encapsuled between the ‘fat bodies’ of the meal-worm. It then undergoes an ecdysis, and, if the meal-worm with its parasites is now eaten by the mouse, the parasites leave their capsule and develop into the sexual form.
As examples of life histories in which a free state intervenes before the intermediate host, Cucullanus elegans and Dracunculus may be selected. The adult Cucullanus elegans is parasitic in the alimentary tract of the Perch and other freshwater fishes. It is a viviparous form, and the young after birth pass out into the water. They next become parasitic in Cyclops, passing in through the mouth, so into the alimentary tract, and thence into the body cavity. They soon undergo an ecdysis, in the course of which the œsophagus becomes divided into a muscular pharynx and true glandular œsophagus. They then grow rapidly in length, and at a second ecdysis acquire a peculiar beaker-like mouth cavity approaching that of the adult. They do not become encapsuled. No further development of the worm takes place so long as it remains in the Cyclops, but, if the Cyclops is now swallowed by a Perch, the worm undergoes a further ecdysis, and rapidly attains to sexual maturity.
The observations of Fedschenko on Dracunculus medinensis[157], which is parasitic in the subcutaneous connective tissue in Man, would seem to shew that it undergoes a metamorphosis very similar to that of Cucullanus. There is moreover a striking resemblance between the larvæ of the two forms. The larvæ of Dracunculus become transported into water, and then make their way into the body cavity of a Cyclops by boring through the soft skin between the segments on the ventral surface of the body. In the body cavity the larvæ undergo an ecdysis and further development. But on reaching a certain stage of development, though they remain a long time in the Cyclops, they grow no further. The remaining history is unknown, but probably the next host is man, in which the larva comes to maturity. In the adult condition only females of Dracunculus are known, and it has been suggested by various writers that the apparent females are in reality hermaphrodites, like Ascaris nigrovenosa, in which the male organs come to maturity before the female.
The sexual form is parasitic in warm climates in the human tissues, and produces multitudes of larvæ which pass into the blood, and are sometimes voided with the urine. The larvæ in the blood do not undergo a further development, and unless transported to an intermediate host die before very long. Some, though as yet hardly sufficient, evidence has been brought forward to shew that if the blood of an infected patient is sucked by a mosquito the larvæ develop further in the alimentary tract of the mosquito, pass through a more or less quiescent stage, and eventually grow considerably in size, and on the death of the mosquito pass into the water. From the water they are probably transported directly or indirectly into the human intestines, and then bore their way into the tissues in which they are parasitic, and become sexually mature.
The well-known Trichina spiralis has a life history unlike that of other known Nematodes, though there can be little doubt that this form should be classified in respect to its life history with the last-described forms. The peculiarity of the life history of Trichina is that the embryos set free in the alimentary canal pass through the walls into the muscular tissues and there encyst; but do not in a general way pass out from the alimentary canal of one host and thence into a fresh host to encyst. It occasionally however happens that this migration does take place, and the life history of Trichina spiralis then becomes almost identical with that of some of the forms of the third type. Trichina is parasitic in man, and in swine, and also in the rat, mouse, cat, fox and other forms which feed upon them. Artificially it can be introduced into various herbivorous forms (rabbit, guinea-pig, horse) and even birds.
The sexual form inhabits the alimentary canal. The female is viviparous, and produces myriads of embryos, which pass into the alimentary canal of their host, through the walls of which they make their way, and travelling along lines of connective tissue pass into the muscles. Here the embryos, which are born in a very imperfect condition, rapidly develop, and eventually assume a quiescent condition in a space inclosed by sarcolemma. Within the sarcolemma a firm capsule is developed for each larva, which after some months becomes calcified; and after the atrophy of the sarcolemma a connective-tissue layer is formed around it. Within its capsule the larva can live for many years, even ten or more, without undergoing further development, but if at last the infected flesh is eaten by a suitable form, e.g. the infected flesh of the pig by man, the quiescent state of the larva is brought to a close, and sexual maturity is attained in the alimentary tract of the new host.
Gordioidea. The free larva of Gordius already described usually penetrates into the larva of Chironomus where it becomes encysted. On the Chironomus being eaten by some fish (Villot, No. [391]) (Phoxinus lævis or Cobitis barbatula), it penetrates into the wall of the intestine of its second host, becomes again encysted and remains quiescent for some time. Eventually in the spring it leaves its capsule, and enters the intestine, and passes to the exterior with the fæces. It then undergoes a gradual metamorphosis, in the course of which it loses its ringed structure and cephalic armature, grows in length, acquires its ventral cord, and on the development of the generative organs loses the greater part of its alimentary tract.
Young examples of Gordius have often been found in various terrestrial carnivorous Insecta, but the meaning of this fact is not yet clear.