OUR DOMESTICATED ANIMALS. O 



spread themselves throughout the eutire substauce of the 

 body. I particularly wish it to be understood that the 

 planarians and flukes are destitute of any geueral somatic 

 or body cavity. The organs in question do not lie loosely 

 in a so-called abdominal space, and for this reason some 

 helminthologists speak of these planarians and flukes as 

 belonging to the parenchymatous or non-cavitary series 

 of worms. The distinction is one of no zoological value ; 

 but in relation to the economy of these creatures it is too 

 important to be passed over in silence. 



As I have employed the term "host," perhaps it is 

 desirable that I should add that the word " guest/' as 

 applied to the entozoon, is equally expressive of the 

 peculiar relation subsisting between the invader and the 

 invaded. Some hosts merely entertain the guests in the 

 capacity of fellow-boarders, in which case the parasites 

 may be looked upon as welcome guests. On the other 

 hand, those entozoa which, like the flukes, remorselessly 

 extract nourishment from the tissues of the bearer must 

 of course be regarded as unwelcome guests. 



In whatever way we view this subject of parasitism, 

 the strangest thoughts and reflections are sure to intrude 

 themselves ; those especially which have reference to the 

 possible mode or modes of origination of these singular 

 creatures being the most difficult to deal with. Amongst 

 the mazes into which a full cousideration of all the facts 

 bearing upon such vexed questions would carry us I do 

 not propose to enter ; but I must ask my readers once for 

 all to abandon the erroneous yet very commonly enter- 

 tained notion, that fluke parasites, as well as other entozoa, 

 are generated as a result of certain diseased or morbid 

 conditions of the tissues of the animal host in which they 

 reside. Not alone by people in general, and by farmers 



