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 eclii- 

 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. 



i. Redia (parent-nurse, Proscolex) produc- 



I 



ing 



5. Cercaria (nurse, Scolex). 



6. Encysted Cercaria (Proglottis). 



7. Distomum (Proglottis). 



The Distomum echiuatuni (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 vulgar is, in whose intes- 



same worm, a species 

 The American 

 ri ed -EW C Ger" oyster is infested by Bucephalus c/rrttlits Ma- 

 vaisandBeneden. cra dy. 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 



Kig. 70 Pro- , ' ,-, j i, , ,, 



iex or parent- tine the adult oi the 

 mum echi>w!m f Gastcrostomum, occurs. 



