21 INTRODUCTION. 



If we follow those Trematoda which are subject to the 

 Alternation of Generations, in their wanderings, we shall see 

 that they are likely to meet with many obstacles to the 

 completion of their developmental course, which is the entering 

 into the viscera of an animal in which they can become sexually 

 developed. It may happen that the various emigrations and 

 immigrations of the infusorial embryo, or of the tailed Cercarice, 

 may miscarry ; or it may be, that the exact time for the Cercaria 

 to become encysted may be missed ; or that after the due occur- 

 rence of the encysting process, the insect selected for its penulti- 

 mate habitation may die at an inappropriate time or place, and 

 so prevent the encysted Cercaria from reaching the last animal, 

 or that one fitted for its final residence. This destruction of the 

 various forms of Trematoda by untoward circumstances is com- 

 pensated by the fact, that they are furnished by the Alternation 

 of Generations with the means of greatly multiplying the various 

 developmental stages of their descendants. By these means the 

 propagation of these animals is secured, since, notwithstanding the 

 mishaps by which many are arrested or destroyed, a sufficient 

 number of individuals always remains out of the numerous young 

 of the nurses and larvae, who, in spite of all obstacles, achieve the 

 end in view the propagation of their species. 



The history of the Cercaria enables us to comprehend many 

 phenomena which were necessarily quite erroneously interpreted 

 by the older helminthologists, who were ignorant of these wan- 

 derings and unacquainted with the occurrence of the Alternation 

 of Generations. It is a common thing to find capsules or cysts, 

 in the midst of the tissues of the most widely different organs of 

 men and animals, containing asexual and only partially developed 

 intestinal worms. It was difficult to understand how such living 

 Entozoa could have originated in the viscera of animals (sometimes 

 in those which are deeply seated and cut off from all external 

 communication) and could here propagate their kind. Hence it 

 was taken for granted that they had been produced by equivocal 

 generation from the surrounding parts, and the mode of origin 

 thus assumed, conversely furnished the reason why these Entozoa 

 were unprovided with sexual organs. Frequently too, free, 

 young, or imperfectly developed intestinal worms were met with 

 in the substance of organs, and their occurrence was in the same 

 way attributed to equivocal generation, though in reality these 

 Entozoa were either in the act of emigrating or of immigrating, 



