TRANSMIGRATIONS AND METAMORPHOSES. 189 



certain fishes; these are the monogenetic trematodes, 

 comprising the Tristomidse and all the worms of that 

 group, which also stand higher in their organization : we 

 shall speak of them hereafter. The other trematodes, 

 which are called digenetic, live on the most dissimilar 

 animals, under the most varied forms, and, like the 

 greater part of the cestodes, introduce themselves into the 

 individual who is to give them shelter, only by the assist- 

 ance of a host, acting as a stage-coach which serves them 

 as a vehicle. 



The principal family is that of the Distomidse, a 

 family par excellence cosmopolitan ; as inconstant in 

 their progress as capricious in the , choice of their com- 

 panions. Each distome resembles a small leech which 

 ha| a sucker in the centre of the belly, and as this 

 sucker was once considered to be perforated, the name 

 of Distoma was given to them. 



These parasites are the more interesting to us, from 

 the fact that, though we are not the final resting-place of 

 certain species, we nevertheless find them pass through 

 us on their way. There are two species which occa- 

 sionally lodge in the liver of man without being peculiar 

 to him, for they properly belong to the sheep. Two 

 other distomes have lately been described by Dr. Bilharz, 

 which are fortunately only known at present in Cairo, 

 and which are interesting, both with respect to their 

 organization and to their manner of life. 



The genealogy of the distomidsB is now generally well 

 known ; that which remains to be discovered is the 

 itinerary of each particular species ; and in several zoo- 

 logical laboratories experiments are daily made with 

 certain species and the hosts which they are supposed 



