CESTOIDEA. 21 



very long time (as yet undetermined) without losing 

 their vitality ; this is not the case with vesicular 

 worms, wliich die very quickly, and often waste away 

 at the end of a few days. 



The embryo having a very different shape from 

 that of the taenia, cannot produce this worm excepting 

 by a metamorphosis or by a new non-sexual gene- 

 ration. It cannot now be doubted that the taenia is 

 reproduced by Alternate Generation, and it is even 

 certain that the life of a taenia comprises more than 

 two phases of generation. These phases are, without 

 doubt, more numerous in certain species than in 

 others ; the tcenia hydatigena, or the tcenia echino- 

 coccus, probably possess one more phase than the 

 taenia which proceeds from a cysticercus. 



The vesicular condition ought to be regarded as 

 one of the larval phases through which a taenia passes 

 before • arriving at the perfect state ; but, has each 

 adult taenia previously been a vesicular worm 1 We 

 may reply with certainty. No ; for, in fact, there are 

 not less tlian two hundred species of perfect taeniae, 

 and there are scarcely more than twenty species of 

 cystic worms. Besides this, the taeniae of herbivorous 

 animals cannot have been taken into the intestines 

 in the vesicular form. The primitive phases of the 

 development of the greater proportion of the taeniae, 

 are as yet entirely unknown. With respect to the 

 adult taeniae, whose vesicular form is supposed to 

 have been decided, they are still small in number. 



In order to arrive at this decision, we judge, 

 partly, by the similitude of the head of a certain 

 vesicular worm to that of a certain taenia ; for 

 instance, that of the cysticercus fasciolar is of the rat 



