In the Digenea the development is indirect, and may involve one or more changes of host, and sometimes a free-swimming stage. The life-history of the liver-fluke of the sheep and other animals may be taken as typical. This form, as an adult fluke, inhabits the bile-ducts of the vertebrate host. The eggs are passed out of the host's body with the faeces, and hatch in a short time in the open if the conditions of moisture and tem perature are suitable. The em bryo, on escaping from the egg, is a ciliated organism known as a miracidium, provided with a pair of eye-spots and an anterior boring organ. It swims about until it meets with a suitable intermediate host (cer tain snails—in Europe, Limnaea truncatula). Boring its way into this through the skin, it sheds its ciliated coat and penetrates into the internal or gans. Here it grows into an ir regular, sac-like body known as a sporocyst. Within this there are formed by budding numerous bodies called rediae. The redia has an oral sucker and a sac-like intestine. Each redia gives rise, by internal budding, either to a further generation of rediae or to larvae of a different type, called cercariae. The cercaria is somewhat tadpole-like, having a broad body and a narrow tail. It has two suckers and a bifurcate intestine. The cercaria escapes from the snail and swims or wriggles about in water, finally coming to rest on some solid body, such as a blade of grass, where it loses its tail and secretes round itself a Should this be swallowed by a suitable vertebrate animal, the cercaria is liberated from the cyst and migrates into the body-cavity and thence into the liver, where it grows into an adult fluke.
An interesting modification of the life-history is found in the genus Leucochloridium. The sporocyst of this form sends branches into the head and tentacles of its snail host. The branches are brightly coloured and capable of pulsating move ments, making the snail conspicuous and particularly liable to the attacks of birds. The transference of the cercariae contained within the snail to suitable final hosts among the birds is thus ensured.
In some genera the cercariae, instead of remaining passive until swallowed, swim about in water and penetrate actively through the skin either of the final or of a second intermediate host. The fork-tailed cercariae of the blood-flukes (Schistosoma) thus attack man and other warm-blooded vertebrates, and the adults inhabit the blood-vessels. The human lung-fluke, Paragonimus, leaves its first intermediate host (a mollusc) as a cercaria, and enters certain freshwater crabs and crayfishes, in which it becomes encysted. Human infection, in the Orient, is acquired by eating these animals. Several small flukes belonging to the genera Opisthorc/iis, Clonorchis and Metagonintus make use, in a similar manner, of freshwater fishes as second inter mediate hosts, the first host being always, so far as is known, a snail.
Economic Importance.—Schistosomiasis (or Bilharziosis) is probably the most important of the human diseases caused by Trematodes. In Egypt and many other parts of Africa two species of Schistosoma, S. haema tobium and S. mansoni, are prevalent. The former also oc curs in Asia, certain localities in the south of Europe, and Aus tralia, and the latter in the West Indies and South America. In the Far East their place is taken by a third species, S. japonicum. These flukes inhabit the mesen teric and portal veins, and their eggs cause obstruction and rup ture of the capillaries either in the wall of the bladder or in that of the bowel, with consequent haemorrhage and ulceration.
The common liver-fluke (Fas ciola hepatica) is an important parasite of sheep, sometimes causing serious and fatal out breaks of the disease known as As a human parasite this worm is rare, but certain smaller liver-flukes and Clonorchis) are not uncom mon in eastern countries.
The lung-fluke, Paragonimus westermani, is a human parasite of some importance in the Far East, and occurs also in South America and Mexico. The various species of flukes which occur in the alimentary canal of man and domestic animals appear to be, on the whole, of relatively little economic significance. (H. A. B.)