FORMS INJURIOUS TO HUMAN INDUSTRIES 361 



Liver- Fluke {Fasciola hepatica, fig. 1257), a brief account of which 

 has already been given (see vol. i, p. 443). The adult Fluke is 

 parasitic in the liver of the sheep, causing the serious disease 

 known as " liver-rot ", while certain earlier stages of the life- 

 history are passed within a small water- snail i^LirnncBa trun- 

 catula). The following extract from Gamble (in The Cambridge 

 Natural History) will give some idea of the serious losses which 

 may be caused by this destructive parasite: — "Over the whole of 

 Europe, Northern Asia, Abyssinia, and North Africa, the Canaries, 

 and the Faroes the fluke and the snail are known to occur, and 

 recently the former has been found in Australia and the Sandwich 

 Islands, where a snail, apparently a variety of Limncea trtmcatttla, 

 is also found. Over these vast areas, however, the disease usually 

 only occurs in certain marshy districts and at certain times of the 

 year. Meadows of a clayey soil, liable to be flooded (as in certain 

 parts of Oxfordshire), are the places where this Limncea occurs 

 most abundantly, and these are consequently the most dangerous 

 feeding -grounds for sheep. The wet years 1816, 181 7, 1830, 

 1853, and 1854 — memorable for the occurrence of acute liver-rot 

 in England, Germany, and France — showed that the weather also 

 plays a considerable part in extending the suitable ground for 

 LimncEa over wide areas which in dry years may be safe pastures. 

 In 1830 England lost from this cause one and a half million sheep, 

 representing some four millions of money, while in 1879-80 three 

 millions died. In 1862 Ireland lost 60 per cent of the flocks, and 

 in 1882 vast numbers of sheep perished in Buenos Ayres from 

 this cause. In the United Kingdom the annual loss was formerly 

 estimated at a million animals, but is now probably considerably 

 less." This extract clearly shows the importance of scientific re- 

 search to agriculture, as preventive measures clearly depend upon 

 an accurate knowledge of the life-history of the fluke. 



Tape -Worms (Cestoda). — The disease of sheep known as 

 "staggers" or "sturdy" is due to the presence of large cysts 

 that cause pressure on the brain, and are the bladder -worm 

 stage {Cosnunts cerebralis) of a Tape -Worm {Tmiia cmnnrtts) 

 that lives when adult in the intestine of a sheep-dog (fig. 1258). 

 A sheep contracts the disease by swallowing- eggs of the parasite 

 which have passed out of the body of a dog, while in its turn a 

 dog becomes infected if it devours cysts from the brain of a sheep 

 that has died of staggers. 



