52 DEVELOPMENT OF 



CHAPTER IV. 



THE DEVELOPMENT OE THE TREMATODA. 



The best known form of tlie numerous family of the 

 Trematoda is \\\.Qßuke or Liverworm {Bistoma hepaticum,) 

 an entozoon which inhabits the hvers of sheep in parti- 

 cular situations, and, especially in wet years, causes much 

 sickness among those animals. Similar forms are met 

 with in almost all animals of the four higher classes, and 

 among the lower, the Mollusca are equally infested by 

 them. 



It might almost be said, that in these classes every 

 species of animal is inhabited by its own fluke ; in several 

 animals also, several different species of these parasites 

 have been found, which inhabit either all the organs of 

 the animal indiscriminately, or are exclusively confined to 

 one, (Liver, Kidneys, Bladder,) or to a definite part of an 

 organ. Several of these Trematoda, as we shall also see, 

 when young are not connected with any organ, but enjoy 

 the power of free locomotion in water externally to the 

 animal which in their future state as entozoa they infest. 

 In this condition they are provided with a locomotive 

 organ, usually a tail of moderate length, by the waving 

 movement of which the creature propels itself through the 

 water, like a tadpole, to which in its external form it is 

 not dissimilar, except that it is much smaller, and almost 

 microscopic. In this larval state, the Trematoda are 

 known to naturalists under the generic name of Cercaria. 



