18 INTRODUCTION. 



sites, belonging to the order Trematoda, bring forth living 

 young, which assume the form of Infusoria, and swim about in 

 the water by means of the cilia which cover the whole surface 

 of their bodies. After some tjme, I observed that these embryos 

 apparently died, their bodies seeming to break up and gradually 

 disappearing, but always leaving behind a sharply defined, mobile, 

 cylindrical body, provided with two short, lateral processes. In 

 all the embryos, without exception, this body was visible through 

 their parietes while they still lived. To my great astonish- 

 ment, upon further observation of these contractile remains of 

 the Monostomum embryos, I discovered that they agreed precisely 

 in form, structure, and movement with certain young Cercaria- 

 sacs. Hence I ventured to conclude that the Cercaria-sac$ pro- 

 ceed from Trematoda. At the same time, these observations 

 seemed to indicate how it was possible for the inert, helpless 

 Cercaria-s&cs to make their way into snails and mussels. The 

 Monostomum mutabile is known to reside in such cavities of the 

 body of wading and swimming birds, as possess natural external 

 apertures ; when, therefore, the embryos of a Monostomum muta- 

 bile are born, they will issue without much difficulty from the 

 animal infested by their parents, each carrying its Cercaria-sac 

 within its body : and the habits of the infested animals are 

 usually such that the embryos will at once pass into water, 

 in which they can, by means of their cilia, swim swiftly about. 



In this element, the infusory Monostomum embryos will, in- 

 stinctively and immediately, seek out those animals that are fit to 

 serve as a nidus for the further development of the Cercaria- 

 sacs enclosed within them. After the Cercaria-sacs have thus 

 passively entered, by the natural apertures, into their appropriate 

 animals, their carriers, the ciliated embryos which have hitherto 

 enclosed them, die off. As a sort of animated covering to the 

 Cercaria-sacs, they have performed their office ; and it is now left 

 to the young that have just been released, to work themselves 

 deeper into their new habitation by their own efforts, and to 

 seek out those places which will afford them the necessary 

 nourishment for further growth, and for the development of their 

 brood of Cer caries. 



I have not yet been able absolutely to witness this process of 

 immigration of Monostomum embryos containing Cercaria-s&cs, 

 and as I have filled up the gaps in observation with my own 

 ideas on the subject, what really occurs may be somewhat 



