ORIGIN OF INTESTINAL WORMS. 27 



which occurs encysted in the peritoneum of the Mole, must also 

 be awaiting its transference to the intestine of some other anial, 

 where it attains its sexual maturity. 



From what has been stated we gather, that those young intestinal 

 worms which are developed at a distance from the nidus of their 

 parents, succeed, in the end, in reaching those situations where 

 they may repeat the part of their progenitors, and reproduce their 

 kind. Impelled by instinct, the embryo parasites, that have only 

 just left the egg, disperse in all directions, so that they may 

 immigrate into other animals, whenever an opportunity offers. 

 Many thousands of these embryos of necessity never attain their 

 object, on account of the numerous casualties that beset them in 

 their wanderings. The point of most importance is, that these 

 embryos should select, as their temporary residence, such crea- 

 tures as will be consumed by those animals, whose intestine served 

 their parents, as a habitation and birth-place for their young. 

 But many of these young, immigrated, intestinal worms will 

 have undertaken their journey in vain, and will die without reach- 

 ing the last stage of their development, in consequence of their 

 host and involuntary carrier, escaping from his natural enemies. 

 Again, many embryos will be led astray by the migratory im- 

 pulse, and pass into animals which never become the prey of those 

 whose digestive canal is their goal. This I conclude from the 

 frequent occurrence of one and the same kind of encysted para- 

 site amongst the most various kinds of animals. And I shall 

 regard those embryos which have failed in their object, in the 

 way I have mentioned, as parasites which have strayed in their 

 wanderings. 



I know that there may be some difficulty in accepting this 

 theory of strayed parasites ; it will be urged that these, like all 

 animals, have a sort of instinct implanted in them which never 

 allows them to enter upon any fruitless undertaking, and which, 

 without their knowing it, impels them to strike out the right 

 path in their wanderings. If this were really the case, every 

 tsenioid embryo must some day become a tape-worm, and we 

 should be so overrun with nernatoid worms that, judging from 

 the enormous quantity of their eggs, the animals they infest 

 would perish by wholesale from their countless numbers. Those 

 who have occupied themselves with the collection of intestinal 

 worms, must only too frequently have remarked, however, that these 

 parasites are by no means so numerous as the immense numbers 



