FLUKES 



443 



each of which is provided with three pairs of hooks, bore their 

 way into the blood-vessels and are carried to the muscles, where 

 they pass into a quiescent stage (pork " measles "). If a pig so 

 infested is slaughtered, and some of the " measly " pork eaten in 

 an imperfectly cooked condition by a human being, the life-history 

 of the parasite is continued a step further, for the " measles " 

 develop in the intestine of the new host into adult tape-worms. 



CHIEF NERVE 



NERVE RING 



ENTRAL SUCKER 



CLASS II. FLUKES (TREMATODA) 



Flukes, like Tape-Worms, are parasitic forms, but they present 

 a wider range in the nature of their parasitism, some being found 

 attached to the exterior of other animals, while some live within 

 various organs of the host. Here 

 again it will, for the present, suffice to 

 describe a single member of the group, 

 and the type which is most commonly 

 taken as a sample is the Liver -F hike 

 (Fasciola hepatica) (fig. 273), a much- 

 dreaded agricultural pest which gives 

 rise to the disease of sheep known as 

 " liver rot ". 



The adult Liver -Fluke is found 

 often in large numbers in the liver of 

 the sheep, or, in some cases, of other 

 animals, among which Man must be 

 included. The body is leaf-shaped and 

 about an inch long. At the front end 

 there is a conical projection on the tip 

 of which is a sucker surrounding the 

 opening of the mouth. There is a 

 second sucker placed farther back on 

 the under surface of the body, and the 

 excretory organs open by a minute 

 pore at the tip of the hinder end. The 



animal is covered by a thick cuticle beset with minute backwardly- 

 directed spines, which facilitate an onward wriggling through the 

 substance of the diseased liver, which gradually becomes more and 

 more degenerated and broken down, till at last, in bad cases, the 

 sheep dies. 



EXCRETORY DUCT 



^W EXCRETORY 



PORE 



Fig. 273. Liver- Fluke (Fasciola. hepatica), 

 diagrammatic 



