ALTEENATION OF OENEKATIONS. 145 



(381.) In various Cercariae, a copious mucous secretion is observable 

 on the surface of the body even before the loss of the tail, and this 

 secretion apparently increases during the efforts of the animal to cast off 

 this appendage. As soon as the tail has been got rid of, the Cercaria 

 begins, by extending and contracting its body, to turn itself round and 

 round in the same spot. By this sort of movement it makes for itself a 

 circular cavity within the mucus, which gradually hardens, and forms a 

 tough, nearly transparent case around it. This is the noted pupa-state 

 of the Cercaria3, observed first by Mtzsch*, and afterwards by Siebold. 

 The tailless CercariaB remain concealed under their transparent case, 

 which is arched over them like a small, closely-shut watch-glass (fig. 72,2) . 

 In this condition they remain some months in a quiescent and inactive 

 state, when they present themselves with all the characters of real 

 Flukes (fig. 72, 3), and may be found under this form lodged in the 

 liver or appropriate viscera in the interior of the snail. 



(382.) Having thus seen that the Cercaria becomes an actual FluTce, 

 it next remains to inquire what is the origin of the Cercaria. The 

 Fluke deposits ova, from which, either within the body of the parent, 

 or external to it, oval-shaped young proceed, which move about briskly 

 in the fluid contained in the interior of the snail or in the surrounding 

 water, and bear no resemblance to their parent. In what way this 

 progeny is transformed into a Fluke, or rather, as we now know, into a 

 Cercaria, is as yet an unexplained mystery ; but that this change can 

 and does take place only by the intervention of several generations may 

 be assumed as beyond doubt. 



(383.) The free-swimming CercariaB, afterwards converted into pupa9, 

 as above described, have been proved, by the observations of Bojanus, to 

 be produced from little worms of a bright yellow colour (fig. 72, 4) 

 (" konigsgelben Wurmern "), described by him, and which occur in 

 great numbers in the interior of snails, especially of Limnceus stagnalis 

 and Paludina vivipara. It is consequently in these yellow worms, 

 which are about 2 lines long, that the CercariaB, which are the Iarva9 of 

 the actual Flukes, are developed ; and since we now know that the 

 Flukes are perfect animals, which themselves undergo no transforma- 

 tion and are propagated by ova, we are reduced to the conclusion that 

 the progeny is indebted for its origin and development to creatures 

 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 meet with a generation of nurses, and 

 that the yellow cylindrical worms of Bojanus, which inhabit the snail, 

 are the nurses of the Cercaria3 and Distomataf. 



* Beitrag zur Infusorienkunde, oder Naturbeschreibung der Zercarien und Bacil- 

 larien. Halle, 1817. 



t That the Cercarice are actually developed in the above-mentioned yellow worms, 

 any one may be easily convinced who will take a dozen large specimens of Limn&us 



L 



