196 ANIMAL PARASITES AND MESSMATES. 



tomian two distinct forms, which begin and end in the 

 same manner, the first putting forth a progeny by means 

 of buds, the second by eggs. There is alternation of 

 form, on account of the double multiplication (digenesis) 

 and migration through several individuals. In other 

 words, the young distome, before it reaches its destina- 

 tion, must change its train many times, and it wears in 

 each carriage a different costume. We can easily under- 

 stand how difficult it is to recognize this travelling dis- 

 tomian, as it changes continually its railway-train and 

 its dress, and what sagacity must have been employed 

 by naturalists in order not to lose its track. 



We may give more than one description of the dis- 

 tomian embryo as it leaves its sporocyst. Is it a mother 

 and an enclosed daughter, as is the case with aphides, or 

 is the ciliated envelope merely a cloak ? We think that 

 the latter is the true interpretation. The ciliated mantle 

 which the embryo loses, is a skin which has been thrown 

 off in monlting, a simple effect of age. 



Thus we find in the complete evolution of a distome 

 an organic and a sexual age, a true alternation ; the 

 agamous age undergoes a true moulting, the sexual age 

 a metamorphosis. 



We have before considered the embryo as mother and 

 daughter coming into the world together, as we see 

 among the aphides ; or the mother, daughter, and grand- 

 daughter are born together like twins ; so that if the 

 mother or the daughter meet with an accident during 

 parturition, the granddaughter may be born before her 

 mother, and even before her grandmother. 



We are now about to study some of these mysterious 

 travellers which have given so much trouble to natural- 



