XXU INTRODUCTION. 



in free communication with the exterior ; they have a 

 horror of being enclosed, and the propagation of their 

 species requires access to the outer air. 



In the last category are found those which need 

 assistance all their lives ; as soon as they have pene- 

 trated into the body' of their host, they never remove 

 again, and the lodging which they have chosen serves 

 them both as a cradle and a tomb. 



Some years since, no one suspected that a parasite 

 could live in any other animal than that in which it was 

 discovered. All helminthologists, with few exceptions, 

 looked upon worms in the interior of the body as formed 

 without parents in the same organs which they occupy. 

 Worms which are parasites of fish, had been seen a 

 long time before this in the intestines of various birds : 

 experiments had even been made to satisfy observers of 

 the possibility of these creatures passing from one body 

 to another; but all these experiments had only given a 

 negative result, and the idea of inevitable transmigration 

 was so completely unknown that Bremser, the first hel- 

 minthologist of his age, raised the cry of heresy, when 

 Eudolphi spoke of the ligula of fishes which could 

 continue to live in birds. 



At a period nearer to our own times, our learned 

 friend, Von Siebold, deservedly called the prince of hel- 

 minthologists, was entirely of this opinion, and com- 

 pared the cysticercus of the mouse with the tape-worm 

 of the cat, considering this young worm as a wandering, 

 sick, and dropsical being. 



In his opinion, the worm had lost its way in the 

 mouse, as the taenia of the cat could live only in the cat. 

 Flourens considered it a romance when I myself an- 



