150 



ZOOLOGY. 



the next form which it assumes the young MoDostomum 

 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 echi- 

 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 : 



Egg. 



Morula. 



Ciliated larva. 



Eedia (parent-nurse, Proscolex) produc- 



1. 

 2. 

 3. 



4. 



ing 



5. 



6. 



7. 



Cercaria (nurse, Scolex). 

 Encysted Cercaria {Proglottis). 

 Distomum {Proglottis). 

 The Distomum eclnnatum (Fig. 100), living 

 in snails which are eaten by ducks, hare 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- 

 tine the adult of the same worm, a species 

 of Oasterostomum, occurs. The American 

 oyster is infested by Bucephalus cuculus Ma- 

 crady. It infests the ovary of the oyster. 

 Whether it is permanently injurious to the latter is un- 

 known. 



Fasciola hepatica (Fig. 101), 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 hy if. 



It is most abundant in sheep in the spring, several hundred 



Fig. 100.— Pro- 

 scolex or parent- 

 nurse of Disto- 

 mum echivMum 

 filled with cerca- 

 rise. — From Ger- 

 vaisand Beneden. 



