110 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). 
Sporocyst (by division). Redia (Snail). 
= a | 
tedia (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 caleu- 
lated that one million sheep are annually lost in the United 
Kingdom from what the farmers call “liver rot” alone. 
The TreMAToDA 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 ectoparasitre. 
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 gills 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 
