

SECTION V. PHYLUM PLATYHELMINTHES 



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 Ccelenterata, 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. 63), with a triangular 

 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, 

 Man. Zool. K 



