212 METAMORPHOSES OF MAN 



ring among the Diptera, and the most important result 

 of which is the appearance of a double reproductive 

 apparatus. The Distomum gradually acquires its per- 

 fect form, and all that remains for it to do is to burst 

 its shell, and adopt the peculiar mode of life for which 

 it is destined.* 



Here we perceive all the characters of typical genea- 

 genesis, complicated, however, by phenomena which 

 are related to metamorphosis properly so called. 



From each ovum there is produced a ciliated larva, 

 which gives rise by internal gemmation to a Spo- 

 rocyst. The latter, by the same process, produces 

 simultaneously new Sporocysts and Cercarise, that is 

 to say, generations which are sometimes less, some- 

 times more advanced in point of development. More- 

 over, each Cercaria passes through stages which may 

 be compared to those which mark the evolution of an 

 insect. It is at first free, and endowed with powers 

 of motion like the larva of the Stratiomys. It becomes 

 encysted like the latter, and by a very analogous 

 process ; and it becomes motionless, and passes, so to 

 speak, into the chrysalis condition. Then it under- 

 goes an organic remodelling, resembling in every 

 respect that by which the nymph is metamorphosed 

 into the perfect insect. Finally, in both cases the 

 termination of these changes is marked out by the 

 development of the apparatus which ensures repro- 

 duction by ova. 



If we apply to the Trematoda the same nomen- 

 clature which we employed in the case of the other 

 groups of which we have treated, we shall say that 



* According to Steenstrup, the Cercaria, prior to assuming their 

 definitive features, remain encysted for nine or ten months. 



