CHAPTER V 



THE PARASITIC WORMS 



In this chapter we shall digress from the Coelomata to deal with three 

 groups of worms — the Trematoda, the Cestoda and the Nematoda — the 

 evolutionary relationships of which with the other groups of the animal 

 kingdom are quite obscure but which are of special interest in view of 

 their mode of life as parasites within the bodies of other animals. 



Trematoda 



An excellent example of this important group of parasites is the Liver- 

 fluke — Fasdola (or Dtsioma) hepatica — which inhabits the bile-ducts of 

 the sheep and is the cause of " Liver Rot/' a source of considerable loss 

 to sheep farmers in marshy, badly drained, districts. 



A fully developed fluke (Fig. 74, H) is a flattened leaf-shaped creature 

 about 30 mm. in length. It is covered by a distinct tough cuticle with 

 innumerable backwardly projecting points so that any relative movement 

 taking place between it and the surrounding tissue tends to make the 

 fluke slip forwards. At the pointed anterior end is the small opening 

 of the mouth, situated at the bottom of a muscular cup — the anterior or 

 oral sucker {o.s). A little way behind the posterior limit of the pointed 

 front portion, and right in the middle line of one of the flat surfaces, is 

 a second, larger, ventral sucker (p.s) by which ordinarily the fluke adheres 

 to the wall of the bile-duct in which it hves. 



The mouth-opening leads into a flask-shaped muscular pharynx by 

 which the blood on which the fluke feeds is sucked in and forced onwards 

 into the intestine. This latter constitutes the remaining and by far the 

 larger portion of the alimentary capal. The intestine has the form of 

 a much elongated (1 — the pharynx leading into the curved portion and 

 the straight limbs running back parallel to one another towards the hind 

 end of the body where they end blindly. Each limb gives off numerous 



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