OUR UNBIDDEN GUESTS. 



79 



obtains fluid food, already digested, the probabilities 

 are that its digestive system will become rudimentary. 

 Not requiring eyes or other sense-organs, these will 

 disappear ; and thus we see represented a kind of 

 zoological backsliding, which reduces the parasite to 

 the elementary and degraded condition we, as a rule, 

 discover in the races of animal " guests/' 



The histories of some of the most common parasites 

 are fraught with instruction, not to speak of the 

 curiosity that invests them. Take, for instance, the 

 history of the fluke (Fasciola hepatica), found in the 

 bile-ducts of the liver of the sheep and ox. It is the 

 presence of this parasite that makes sheep fall into a 

 decline, known to veterinarians as the "rot." A 

 fluke is a little, flattened, oval body, about 1 in. 

 or f in. in length, and about ^ in. in breadth. It 

 possesses a nervous system, a set of water -vessels, 

 two suckers, a branched digestive system, and an egg- 

 producing apparatus. It has no organs of motion, 

 but it is by no means a very degraded being after all. 

 Its development is very curious. The eggs, liberated 

 from the animal " host," get scattered abroad. Many 

 -as in the case of all parasites mnst perish, but a 

 proportion finding their way into water, enter the body 

 of the water-snail, where they develop into curious 

 little tailed beings called Cercarice; and there are 

 sundry other forms assumed by the fluke in the days 

 of its youth, but which need not be mentioned here. 

 Sooner or later, however, these Cercarice escape into 

 the water or into the meadows ; and it is believed that 



