62 DEVELOPMENT OF 



and interpret the phenomena thus presented in a natural 

 way, we may to a certain extent penetrate the obscurity 

 which involves the solution of this enigma, although we 

 shall not arrive at its complete elucidation. 



Thus if we are desirous of tracing the whole series of 

 development in this species of fluhe, in the absence of aU 

 observations upon its ova and their progeny, we can only 

 recur to their earher stages of development, and try 

 whether we . cannot trace them up to the egg, or at least 

 as near as may be to that point. 



Whence comes then the free swimming Cercaria^ after- 

 wards a pupa, viz. the Cercaria echinata? 



Tliis question is answered by the observations of Boja- 

 nus, who states, that this species is the same with those 

 he saw swarming out from the " king s-yellow worms'^ 

 (" konigsgelben Wüi^mern") described by him, and which 

 occur in great numbers in the interior of snails, especially 

 of LimncBus stagnalis and Paludina vivipara. It is, con- 

 sequently, in these yellow worms, which are about two 

 lines long, that the Cercarice which are the larvcR of the 

 2jdu(sA flukes are developed ; and since we now know that 

 the flukes are perfect animals, which themselves undergo 

 no transformation and are propagated by ova, we are 

 reduced to the conclusion that the progeny is indebted 

 for its origin and development to animals, which in 

 external form and partly in internal organization, differ 

 from the animals into which that progeny is afterwards 

 developed ; in other words it may be said, that we here 

 again meet with generations of " nurses,'' and that the 

 yellow cyHndrical worms of Bojanus which inhabit the 

 snail, are the " nurses' of the CercaricB and JDistomata. 

 That the Cercarice are actually developed in the above- 

 mentioned yellow worms, any one may be easily con- 

 vinced who will take a dozen large specimens of Lhnnceus 

 stagnalis from small stagnant pools which have been ex- 

 posed to the sun ; the worms will be very readily found. 

 They are situated not so much in the viscera themselves, 

 (the liver and reproductive organs,) as in the membranes 



