SECTION V. — PHYLUM PLATYHEL- 

 MINTHES 



The Platyhelminthes or Flat-worms are a group of animals 

 which, though of a low type of organisation, yet show in 

 many cases a great advance on the Coelenterata, in the 

 possession of systems of organs of a more or less elaborate 

 character for the carrying on of the various functions. 

 Many are internal parasites of higher animals ; others are 

 parasites on the outer surface (external parasites) ; others 

 again are non-parasitic. 



1. THE TREMATODA 



A good and easily procurable example of the flat-worms 

 is the Liver-fluke of the sheep (Distomum hepaticum), 

 which lives as a parasite in the liver, in the interior of the 

 larger bile-ducts of the infested animal. It is a soft-bodied 

 worm, of flattened, leaf-like shape (Fig. 68), with a trian- 

 gular process, the head lobe, projecting from the broader 

 end. When the liver-fluke is compared with a zooid of Obelia, 

 or with a Medusa or a sea-anemone, a striking difference in 

 the general disposition or symmetry of the parts is at once 

 recognisable. In the latter, as in the Ccelenterata in general, 

 the prevailing arrangement is a radial one, the parts being 

 disposed in a radial manner round the main axis of the body, 

 which is an imaginary line running through the middle of 

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