6 INTRODUCTION. 



physicians and naturalists have devoted the requisite attention to 

 the subject of these wanderings (to which I directed attention 

 some years back 1 ), a number of facts have been discovered, show- 

 ing more and more that the origin of the intestinal worms in the 

 viscera of animals can be readily accounted for according to 

 natural laws; whereas formerly, hardly anything being really 

 known of the natural history of these parasites, their mode of 

 origin and propagation, already difficult enough of comprehension, 

 was rendered more and more mysterious by an hypothesis of 

 " equivocal generation" entirely devoid of any direct support. 



An important circumstance, very favorable to the progeny 

 of the intestinal worms during their wanderings, is the solidity 

 of the egg-shell in which they are commonly contained. By 

 its hardness and resistance, the egg-shell of many kinds of 

 intestinal worms efficiently protects the enclosed germ and 

 yelk, or the already developed embryo, against injury from 

 without, and maintains within the ovum the degree of moisture 

 requisite for the further development of the young. In this 

 way the ova preserve their vitality for months together, not- 

 withstanding the many vicissitudes to which they are exposed 

 after leaving the dwelling of their parents. They pass into 

 dust-heaps, privies, drains, &c., where, surrounded sometimes by 

 a greater, sometimes by a lesser degree of moisture, they are 

 subjected to various degrees of temperature, until, deposited 

 in the dung-heaps into which corrupt and mouldering organic 

 substances are usually converted, they are, as manure, spread 

 upon the fields and meadows, where, under favorable in- 

 fluences of the weather, particularly if supplied with adequate 

 moisture, they become further developed. It will be obvious that 

 the young of the intestinal worms have not far to seek for an 

 opportunity of re-entering other animals, when we consider that 

 they are scattered through the manured soil amongst the seeds 

 that have been sown there; that these produce plants which 

 generally serve for the support of men and animals, and that the 

 young worms adhering to them may thus be easily swallowed. 

 Again, it may well happen that showers of rain occasionally 

 wash out the ova of the intestinal worms from the dung-heaps 

 or manured soil, carrying them off into streams arid brooks, and 



i See my article " Parasiten" in R. Wagner's ' Handworterbuch der Physiologic,' 

 Bd. ii, 1844, p. 645. 



