150 



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 : 

 I- Egg. 



2. Morula. 



3. Ciliated larva. 



4. Eedia (parent-nurse, Proscolex) produc- 

 ing 



5. Cercaria (nurse, Scolex). 



6. Encysted Cercaria (Proglottis}. 



7. Distomum (Proglottis}. 



The Distomum echinatuin (Fig. 100), 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 vulqaris, in whose intes- 



Fig. 100. Pro- ,- n i ij. 4 4.1 



ecoiek or parent- tine the adult of the same worm, a species 

 rnum eckiiiafum of Gasterostomum, occurs. The American 

 rte^^rom^er- oyster is infested by Bucephalus cuculus Ma- 

 vaisand Beneden. cra dy. It infests the ovary of the oyster. 

 Whether it is permanently injurious to the latter is un- 

 known. 



Fasciola liepatica (Fig. 101), the liver-fluke, sometimes 

 occurring in man, is thought by Dr. Willemoes-Suhm to 

 begin its existence as Cercaria cystophora, parasitic on a 

 species of Planorbis. 



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





