ZOOLOGY 



into the water (Fig. 73, E). They then swim about for a time, 

 and ultimately settle on some water-plant, or during a flood 

 on the grass, and by means of the cystogenous cells envelop 

 themselves in a cyst. If they are then swallowed by the 

 grazing sheep, they make their way to the bile duct, and there 

 develop into the sexual Fasciola hepatica. It is thus evident 

 that in the life-history of this Trematode there is an alterna- 

 tion of generations, during which there are several occurrences 

 of asexual reproduction. 



Life-History of Fasciola. Hepatica. 



Sexual Adult (Sheep). _ 

 Ciliated Embryo (Water and Snail). 

 Sporocyst (Snail). 

 Sporoeyst (by division). Redia (Snail). 

 Redia (by gemmation). Cercaria (encysted on grass). 

 Sexual Adult (Sheep). 



The disastrous effects which this internal parasite pro- 

 duces on its host are evidenced by the fact that it is calcu- 

 lated that one million sheep are annually lost in the United 

 Kingdom from what the farmers call " liver rot " alone. 



The Tkematoda are divided into the Monogenea and the 

 Digenea. 



A. The Monogenea develop directly, without the intervention 

 of asexual forms. They inhabit therefore one host only, 

 and are with few exceptions ectoparccsitic. 



Amongst the exceptions to the last statement, is the species 

 Aspidogaster conchicola, which inhabits the pericardial cavity 

 and the nephridia of the freshwater mussel (Anodon) (Fig. 74). 



A very curious Trematode inhabits the gills of the 

 minnow. Its embryo is ciliated and free-swimming, and is 

 termed a Diporpa ; it, however, soon loses its cilia and settles 

 down on the giUs of its host. At first it lives singly, but 

 after a time two individuals come in contact, and one seizes 

 the dorsal papilla of the other by its ventral sucker; they 

 then twist round so that the ventral sucker of the second is 



