TRANSMIGRATIONS AND METAMORPHOSES. 



191 



ever see lier retrograde tlius from a more complicated 

 organization to one more simple ?" That which this great 

 observer did not dare to believe has yet been realized, 

 and in many cases development is clearly recm-rent. 



Led by a marvellous instinct, and obeying an irre- 

 vocable mission, the distomidse, as 

 well as the monostomidae, and 

 others besides them, when they 

 claim an asvlmn from molluscs, 

 introduce into the living body of 

 their new host, not an isolated 

 embryo, but a 3'Oung animal 

 already impregnated with a rich 

 posterity; if she remam mistress 

 of the situation, this posterity 

 will forcibly invade the various 

 organs, without any consideration 

 whether their host may not give 

 way under the weight of this 

 sudden invasion. 



Fig. 41 represents one of these 

 worms which proceeds from a cili- 

 ated embryo, and encloses by the 

 side of its digestive tube cercariae 

 in different degrees of develop- 

 ment. In front, we see one pro- 

 vided with eyes and a tail ; behind, 

 we see others which are j^ounger ; 

 among these ciliated embr^^os, 

 wandering without guidance and 

 without a compass in the midst of their ocean, but few 



will reach the land, or, in other words, will find the 

 10 



Fig. 41. — ]\[onostoraum verru- 

 cosum, Sporocyst with Cer- 

 carife. In front is the mouth, 

 in the middle the digestive 

 canal, and around the diges- 

 tive canal are young ones, 

 under the form of Cercariaj 

 in process of development. 



