204 



THE ANIMAL PARASITES OF MAN 



takes place in the definitive host. In the cysticercus stages of other 

 tapeworms we can always distinguish the scolex and a caudal- 

 like vesicular or compact appendix. The scolex alone forms 

 the future tap'eworm, the variously-formed appendix perishing. 



It has now been proved that the appendix, the caudal vesicle, 

 originates direct from the body of the oncosphere, and therefore is 

 primary, and that the scolex only subsequently forms through pro- 

 liferation within this appendix. On account of this origin the scolex 

 is generally regarded as the daughter, and the part usually designated 

 as the appendix as the mother, originating from the oncosphefe. 



Accordingly, two modes of development of the cysticercus stage 

 may be distinguished ; in the one case the oncosphere changes 

 direct into the scolex, thus forming the body of the tapeworm within 

 the primary host ; in the other case the scolex only forms 

 secondarily in the transformed body of the oncosphere, which 



FIG. 132. -Plerocercus of Tetrar- 

 hynchus. 20/1. 



FIG. 133. Young cysticerci of Tcenia 

 saginata, G. (magnified). On the left 

 seen from above, on the right optical 

 section with rudiment of scolex. (From 

 Leuckart.) 



later on perishes, the scolex alone remaining as the originator of 

 the tapeworm colony. 



The direct metamorphosis of the oncosphere into the cysticercus 

 form termed plerocercoid, has hitherto not been investigated, 

 although Ligula, Schistocephalus and Bothriocephalus are very com- 

 mon parasites, but many circumstances point to the conclusions 

 arrived at by us and by other observers. 



In a broad sense, our knowledge of the development of the 

 cysticerci is limited almost exclusively to that of a few " cystic 

 worms " ; in other cases we know either only the terminal stage, 

 the complete cysticercus, or, exceptionally, one of the intermediary 

 stages, but we are not acquainted with a complete series ; the 

 description must therefore be incomplete. 



We know from feeding experiments that, after the intro- 



