146 PARASITIC ANIMALS. 



fact, the more mysterious does it appear. The 

 localities thus occupied have led to many conjectures; 

 and How do they get there ? has been often asked, 

 but not often satisfactorily answered. 



Certain it is, that they have been neither detected 

 in the air, nor in the water ; they are not known to 

 exist on grasses or plants, nor on the surface of the 

 ground beneath our feet ; they are found exclusively 

 in situations in which it is difficult to prove how 

 they can have been deposited, yet it is here only, as 

 far as our limited sphere of observation extends, 

 that they are met with, and in no others. To what 

 do we allude ? To those extraordinary creatures 

 which are only known as tenants of the living 

 bodies of other animals, and of which some species 

 are respectively abundant in the fluids of the system, 

 while others occupy the alimentary canal, and others 

 are imbedded in the muscles, the liver, and the 

 brain. They are parasitic animals, and have 

 received the general name of Entozoa, from two 

 Greek words, within, and an animal, from the cir- 

 cumstances connected with their existence ; but all 

 do not belong to the same division of the animal 

 kingdom. Some form part of the group* in which 

 no nervous filaments have been discovered, while 

 others are more highly organized. 



The Trichina spiralis exists only in the muscular 

 structure of the human subject; the Linguatula in- 

 habits the hollows of the frontal bone, called frontal 

 * Acrita. 



