DEGENERATION OF ENCYSTED PARASITES. 



83 



These last words show that the author regarded the asexual 

 bladder -worms as Helminths that had degenerated and become drop- 

 sical, in consequence of having lost their way, and got into the body- 

 cavity or muscles of their host instead of its intestine. 



Bladder- worms (Fig. 57) and encysted Trichinae (at that time only 

 known in the encysted condition Fig. 58) were the only parasites 

 regarded thus by von Siebold, and at that period neither the im- 

 portance nor wide distribution of the encysted condition in the life- 

 history of parasites was understood, the generally received opinion being 



FIG. 57. Measly port (natural size). FIG. 58. Trichinosed pork (enlarged 45 times). 



that the germs migrated immediately into the body of their definitive 

 host. At this period, then, von Siebold's hypothesis was an attempt 

 to explain certain striking and unintelligible facts, but has now be- 

 come out of date. It is just these bladder-worms and Trichince that 

 have become by a remarkable concurrence of circumstances the very 

 subject of experimental investigation, and we are now thoroughly 

 acquainted with their natural history. There is not the slightest 

 doubt that what von Siebold considered to be abnormal conditions 

 are in reality the ordinary stages of development ; that Trichina, before 

 arriving at sexual maturity, always passes through a stage in which 

 it is encysted in muscle, and that in the same way tape-worms are 

 invariably derived from bladder-worms. We can, therefore, lay aside 

 von Siebold's theory, which has now hardly any supporters, in spite 

 of the great reputation of its originator. 



