14 INTRODUCTION. 



physiologically, is, that they bring forth young, without themselves 

 possessing any real sexual apparatus. These Agamozooids, in 

 fact, multiply by division, by external or internal gemmation ; 

 they develop within their bodies germs which become fresli 

 creatures. But these germs dcf not deserve the title of " eggs;" 

 nor is the place where they are developed to be called an 

 " ovarium," since the germs, which I shall for the future 

 distinguish by the name of " sporulae," are not only devoid 

 of the ordinary constituents of an ovum, as vitelline mem- 

 brane, yolk, germinal vesicle, and so called germinal spot, 

 but the further development of the germ-body is not pre- 

 ceded by those conditions, (I mean that " impregnation" by 

 means of a special seminal matter produced in a testis,) which 

 is essential to the development of true ova developed within 

 an ovarium. The organ in which, in certain Agamozooids the 

 "gemmse" are formed, cannot therefore be properly termed an 

 " ovarium," and I shall distinguish it by the name of " sporula- 

 rium." No et nurses" present any sexual distinctions, and hence 

 their method of multiplication and propagation, which takes place 

 by means of sporulse formed within sporularia, or by ordinary 

 budding, or by division, must be arranged amidst the modes of 

 asexual reproduction. 



Very many cases of the alternation of generations occur 

 among the Trematoda. The relations that the various changing 

 forms of these animals have to one another, remained long un- 

 suspected, since it was not an easy matter to discover among the 

 various successively alternating generations of a single fluke- 

 worm, the clue to their origin from one and the same parent. 

 The recognition of the connection of these forms, was ren- 

 dered more difficult of discovery by the fact, that these alternating 

 generations of animals not only changed their appearance, but 

 also their dwellings, whereby their parentage was still further 

 concealed. These multitudinous difficulties in the way of the 

 observers of the alternation of generations, render it impossible 

 for me to give a complete account of all the complex series of 

 changes undergone by any single Trematode in the course of its 

 development. Up to this time only longer or shorter fragments 

 of the circle of vital phenomena, broken as they are into many 

 phases by the alternation of generations, have been made out in a 

 few Trematoda. 



However, these fragments do not relate to one and the same 

 period in the life of these parasites, nor to the same generations 



