Chap, in.] FLAT-WORMS. 



49 





The lowest Metazoa are grouped into a somewhat 

 heterogeneous mob, which is known as the division of 

 the Verities or Worms. Of these the lowest are the 

 Flat-Worms. 



A. Platylielmiiithes. Of the three divisions of 

 flat-worms, two are degraded by parasitism ; such are 

 the divisions to which the tape-worms (Tsenia), and 

 the flukes (Distomum) belong. 



I. The Turfoellaria are the simplest forms, and 

 are free living; the body is soft and small, covered 

 with cilia, and without an anus ; the entrance to the 

 digestive tract is often provided with a proboscis, and 

 the generative apparatus may be simple, or may be 

 greatly complicated. A distinct cceloin is not always 

 apparent (Acoela), or it may become secondarily 

 obscured. The enteric tract is straight, or branched. 

 Planaria, Dendrocoelum, and Mesostomnm are ex- 

 amples of this division. 



II. The Trematoda are flat- worms that have 

 taken to a parasitic mode of life, but are by no 

 means so profoundly modified as the members of the 

 group next to be considered. They either live on 

 the bodies of other animals (ecto-parasitic), like 

 Aspidogaster, which is found in the gill chamber of the 

 fresh-water mussel ; in this case they exhibit no 

 "alternation of generation." Or they live within the 

 bodies of other animals (pento-arasitic), like Disto- 

 mum hepaticum (the liver-fluke) ; in this case they 

 pass different stages of their existence in two different 

 animals. The ciliated covering is lost, and suckers are 

 developed, which serve as organs of attachment, and, 

 to a certain degree also, as organs of locomotion ; the 

 sexes are ordinarily united in the same individual, and 

 the accessory parts of the generative apparatus are 

 greatly complicated. 



III. The Cestoda, or tape-worms, are flat- 

 worms which are still further modified in accordance 

 E 16 



