ii DISTOMUM HEPATICUM 67 



Holostomatidae) has now become, by maturation of the Cercaria 

 in the comparatively modern warm-blooded bird, a metamorphosis 

 extending over two or more generations. 



Distomum (Fasciola) hepaticum The liver-fluke of the Sheep, 

 which produces the disastrous disease, liver-rot, has a distribution 

 as wide as that of a small water-snail, Limnaea truncatula, the 

 connexion between the two being, as Thomas x and Leuckart dis- 

 covered, that this snail is the intermediate host in which the 

 earlier larval, sporocyst, and redia stages are passed through, and 

 a vast number of immature flukes (Cercariae) are developed. 

 These leave the snail and encyst upon grass, where they are eaten 

 by the sheep. 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 Limnaea truncatula, is also found. 2 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 Oxford- 

 shire), are the places where this Limnaea occurs most abundantly, 

 and these are consequently the most dangerous feeding-grounds 

 for sheep. The wet years 1816, 1817, 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 Limnaea 

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

 1830 England lost from this cause, 3 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. After infection during a wet 

 autumn, it is usually in the succeeding winter that the disease 

 reaches its height. 



1 Quart. Journ. Micros. Sci. vol. xxiii. 1883, p. 90. 



- The intermediate host in the Sandwich Islands is said to be Limnaea pcreyra. 

 See Lutz, Ccntralbl. f. Lakter. xi. 1892, p. 783. 



3 The mortality in wet years, however, is said to be largely due to pulmonary 

 inflammation. This and other causes of death are not always discriminated in tho 

 returns. 



