202 METAMOEPHOSES OF MAN 



instinctive feeling of disgust. If the reader desires to 

 follow us, we shall spare him the technical details in 

 endeavouring to convey some portions of a history, 

 which touches on the most important questions in 

 general and philosophic physiology. 



Until of late years the term Helminthes had 

 been exclusively applied to those worms which 

 remain concealed within the bodies of other ani- 

 mals. At present this limitation does not exist. 

 It is known now that these internal parasites have 

 very close relations among external beings. The 

 Nemertes and PlanarisB are nearly akin to. the Trema- 

 toda, of which we shall speak presently. These 

 affinities which have been but recently observed, have 

 caused the Helminthes to be no longer ranged among 

 the Badiata where Cuvier placed them, but along with 

 the Annelida. Consequently, had we been faithful to 

 our scheme, we should have considered these peculiar 

 beings before now ; but it seemed preferable to devote 

 a special chapter to them. The exceptional mode of 

 life of most of them, the very complex phenomena 

 of their development, and the unexpected light which 

 the study of Helminthes has thrown upon some of the 

 most obscure problems in science, amply justify the 

 departure from the order pursued in other portions of 

 this volume. 



From our point of view, the Helminthes which lead 

 an external and independent life, are not of any 

 peculiar interest ; the parasitic species alone merit 

 our attention. The latter have been divided into a 

 certain number of groups, from which we shall select 

 the Trematoda, Cesto'ida, and the Cystic worms. 



The first are usually animals of small size, flat and 



