l^yO _ EOAKD OF AGRICULTURE. 



found developed into Cercaria-sacs, and living in the abdominal 

 cavities or among the reproductive organs of the molluscs which 

 are to furnish them their proper entertainment while they perfect 

 and bring forth their broods of young Cercaria, similar to those 

 M^ith which this curious history began. Now in general, we 

 expect to see offspring resemble their parents, but in the case of 

 Cercaria four generations and one metamorphosis are required 

 before this resemblance is reached. Many other instances very 

 similar are known, in which insects, birds, I'eptiles, fishes, etc. may 

 be concerned. The old practice of shepherds, which did not allow 

 their sheep to go out to graze till the dew had entirely disappeared, 

 would seem to protect them against the fluke, since the grass being 

 dry there is less danger that enyils v.'ill bo adhering to it, and hence 

 less danger that they with their encysted cercaria will be swallowed. 



It is easily seen, too, why high, mountain pastures are fitter for 

 flocks than low, wet ones. 



The principle of Alternate Generation i5nds its illustration in the 

 Cestoidea or tape-worms. The importance of this subject will be 

 a suflBcient reason for presenting some of the principal facts. 



There is often found infesting the livers of rats and mice, an 

 encysted worm, whose destiny was for a long time unknown. « 

 Feeding experiments and careful observation have shown that 

 when they have passed into the stomach of the cat, they do not 

 undergo digestion, but develop into tape-worms, the joints of 

 which becoming sexually matured in tlic intestine, produce vast 

 numbers of eggs, which will in due time be expelled ; and by far 

 the greater part being lost, a few will find their way into the 

 stomachs of other mice or rats, where they will find a fitting home 

 for a few days, after which they actively emigrate to the liver or 

 other organs in which they become encysted only to be liberated 

 by some other cat devouring their host. And this cat thus 

 becomes possessed of tape-worms in like manner. So also in the 

 intestines of the meal-worm — may be found small encysted worms 

 whicli with the worm may be swallowed by the mouse, or by certain 

 birds ; and when liberated by digestion, they find in the intestine 

 of their devourer, the proper conditions for their growth and 

 sexual maturity, which they soon reach as tape-worms, and then 

 are expelled either containing countless numbers of eggs, or hav- 

 ing previously discharged them. These eggs, swallowed by otlier 

 meal-worms, will soon be hatched, and the young cystic worms 

 will migrate by cutting their way to the proper organs, where 



