94 DEVELOPMENT OF 



{d) The Development of other Trematode Animals. 



Thus, if we trace the ciliated trematode embryos in 

 their progress towards a perfect Trematode, or if we 

 proceed in the other direction and go back from the 

 Trematode itself, and endeavour to get as near as possible 

 to its origin, the same phenomena will be presented to us, 

 since the species must, from the egg upwards, necessarily 

 pass through several generations succeeding each other in 

 a definite order before it appears as individuals, which 

 both externally and internally resemble those from which 

 the course of development commenced, and which are 

 capable of propagating the species anew by the genera- 

 tion of ova. That all trematode animals are developed in 

 this way, that aU are obliged to undergo such alternation 

 of generation cannot be directly affirmed from the facts 

 adduced ; these, however, are sufficient to warrant the 

 supposition that this is probably the case, I will here 

 only recall the general observation made with regard to 

 the ova of the Trematoda, that there always proceeds 

 from them a more or less active ciliated animalcule which 

 has not the slightest resemblance to the parent animal, 

 and consequently, under all circumstances, must undergo 

 a remarkable change or metamorphosis. Tt might, on 

 this account, with great probability be assumed, that its 

 progress and course of development corresponded with 

 those of similar creatures, and that it was metamorphosed 

 into a " nursing" individual, as is shown by Siebold's 

 observations on Monostoma mutabile, and mine upon 

 Distoma duplicatum. If such a course of development 

 be supposed, there must be as many forms of "nurses" as 

 there are species of Trematoda, and since such " nursing'' 

 animals have not been observed, it would certainly be 

 extraordinary that all the search after entozoa, which has 

 been made, should not have brought to light animals. 



