210 METAMOEPHOSES OF MAN 



experiments; so that now, thanks to the number of 

 their works, we can trace, if not the special history 

 of each species, at all events, the general history of 

 these beings, which in former days were looked on 

 as such mysteries. 



In the first place, let us treat of the Trematodes. 

 We shall select, as an example, one of the species 

 akin to the Monostomum midabile, or the Bistomum 

 militare* which have been investigated by both 

 Siebold and Van Beneden. 



The general description of trematodes already given 

 will suffice to convey an idea of these animals. They 

 may be described as little leeches, inhabiting the bodies 

 of fresh-water mollusks. In the bodies of these hel- 

 minthes hundreds of ova may be found, whose vitelli 

 have already undergone their first transformations, and 

 have become ciliated larvae. These latter leave their 

 envelopes, and, . thus set free, make their way into the 

 bodies of mollusks by some means or other. There 

 they remain, and appear to undergo destruction, whilst 

 in their places are left a number of very small ovoid 

 bodies, which have been developed within them. 



Bach of these bodies, which were regarded as 

 necessary parasites by the German authors, and as 

 enigmatical organs by F. Dujardin, increases in size, 

 elongates, and acquires two lateral appendages pos- 

 teriorly. This is the sporocyst of Baer, the a redie }> 

 of Filippi.f Judging by its movements, and its well- 



* The Distomuni and Monostomum are genera belonging to the 

 Trematode group. 



t It has been very reasonably proposed by Filippi to distinguish 

 the transitional beings by giving them different names, according 

 as they present a more or less complex organization, or rather 



