OKIGIN OF INTESTINAL WOEMS. 5 



forth, or laid as eggs (that is to say, they emerge from the intes- 

 tine of their parent's host), and seek an opportunity to enter into 

 the intestine of some other creature. It is easy to convince 

 oneself of this emigration of the young of the tape-worm, by 

 examining the excrement of animals infested by them, at those 

 times of the year at which they attain their sexual maturity. 

 We then observe, that sometimes single joints, or connected 

 series of joints, full of ova ; sometimes immense masses of the 

 ova, are passed with the faeces. The same thing holds good 

 with regard to the ova of the Distomata that infest the livers of 

 our ruminating animals ; their eggs, after they have been trans- 

 ferred from the liver to the gall-ducts, being washed out with the 

 bile into the intestine, and evacuated with the dung. 



These emigrations of the young of the intestinal worms benefit 

 not only the creatures they infest, but themselves. There are many 

 kinds of intestinal worms, in whose eggs the embryo is never hatched 

 if they remain in the place where they have been laid. They 

 must wander to some other place in order to develop their young, 

 or to allow of the escape of the young already developed in them. 1 

 These young must then either wait for, or seek, an animal to 

 lodge in, having entered into which, they are capable of attaining 

 sexual maturity. By such emigrations the infested animals are at 

 the same time freed from guests, whose increase would be both 

 troublesome and prejudicial. For example, what would happen 

 if the millions of eggs that a single round-worm or tape-worm 

 can produce, were to develop and generate their young in the 

 same intestine in which they were laid ? Would not the intes- 

 tine, after the young had attained their full growth, and brought 

 forth others in their turn, become at last so choked up as to 

 disable this part of the digestive apparatus, so that the whole 

 organism of the imhappy animal must perish, together with his 

 parasites ? In any case, the emigration and immigration of the 

 young of the intestinal worms, is a very important though long 

 unregarded part of the history of their propagation ; and since 



1 Hence a tape- worm which has found its way into the intestine of an appropriate 

 animal will attain its sexual maturity, but will not, properly speaking, multiply its kind 

 there. For this reason, the tape-worm (Tania solium) infesting the human subject, 

 which is common in Germany, France, [and England,] is commonly called the 

 solitary worm, (Einsiedler-wurm, ver solitaire,) although the name is not a very fit one, 

 as it depends entirely on accident whether only a single individual or a whole society of 

 these worms shall enter the human intestine in the course of their wanderings. 



