108 ZOOLOGY. 
the next form which it assumes the young Monostomum 
bears an undeniable resemblance to those animals which 
I have termed ‘ nurses’ and ‘ parent-nurses’ in that species 
of the Trematoda which is developed from the Cercaria echt- 
nata.”’ 
Thus the cycle is completed, and the following summary 
of changes undergone by the Distomes present as clear a 
case of an alternation of generations as seen’ 
in the jelly-fishes : 
1. Egg. 
2. Morula. 
3. Ciliated larva. 
4. Redia (parent-nurse, Proscolex) produc- 
5. Cercaria (nurse, Scolez). 
6. Encysted Cercaria (Proglottis). 
%. Distomum (Proglottis). 
The Distomum echinatwmn (Fig. 70), living 
in snails which are eaten by ducks, have been 
shown by St. George to develop into the adult 
Distoma in the body of that bird. It is gen- 
erally the case that those Distomes which pass 
through an alternation of generations live in 
the larval state in animals which serve as food 
for higher orders. Thus the Bucephalus of 
the European oyster passes in the encysted 
state into a fish which serves as food for a 
larger fish, Belone vulgaris, in whose intes- 
scolex’or pawns, tine the adult of the same worm, a species 
nurse of ve of Gasterostomum, occurs. The American 
filed with cerca- oyster is infested by Bucephalus cuculus Ma- 
vaisand Beneden. crady, It infests the ovary of the oyster. 
Whether it is permanently injurious to the latter is un- 
known. 
Fasciola hepatica (Fig. 71), the liver-fluke, sometimes 
occurring in man, causes the “liver-rot” in sheep, etc. In 
the winter of 1879-80, it was so prevalent in Great Britain 
that 3,000,000 sheep were destroyed by it. 
It is most abundant in sheep in the spring, several hundred 
