150 



ZOOLOGY. 



the next form wliicli 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- 

 naf.a." 



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. Eedia (parent-nurse, Proscolex) produc- 

 ing 



5. Cercaria (nurse, Scolex). 



6. Encysted Cercaria {Proglottis). 



7. Distomum {Proglottis). 

 The Distomum echinata (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 jjasses in the encysted 

 state into a fish which serves as food for a 

 larger fish, Belone vulaaris, in whose intes- 



Fig. lOO.-Pro- ,■ ,1 J li i ii 



scolex or parent- tmc the adult of the samc worm, a species 

 mum echinatvm ot Gasfcrosfomum, occurs. The American 

 riif'Lprom''Ge?- oyster is infested by Bucephalus cuculus Ma- 

 vaiaandBeneden. ^j.^dy. It infests the ovary of the oysteT. 

 Whether it is permanently injurious to the latter is un- 

 known. 



Pasciola hepaticum (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 



