CHAPTER V 



PLATYHELMINTHES 



Classification adopted 



Platyhelminthes 



Turbellaria 



Trematoda 



. Cestoda 



_. , . f Polyclada 

 f Dendrocoela { Triclada 



| I Acoelida 



Ehabdocoela -j Rhabdocoelida 

 [ Alloiocoelida 



THE phylum of Platyhelminthes or Flat -worms agrees in one 

 point of structure of great importance with the Coelenterata. The 

 alimentary canal may, it is true, entirely disappear in some members 

 of the phylum which are internal parasites and which live Ly the 

 absorption of fluids through the skin, but when it is present it has 

 only a single opening, the mouth, which serves both for ingestion 

 and egestion. Further, between alimentary canal and skin there is 

 no real body-cavity, the space being occupied by a ground substance 

 comparable to the jelly of a Coelenterata. This ground substance is 

 invaded by numerous stellate cells connected together by their pro- 

 cesses, some of which become muscular and contractile ; in these 

 two respects this tissue resembles especially the jelly of a Ctenophore. 



When we survey the development of the eggs of this group, we 

 find a feature common to a large number which entails modifications 

 of the early life -history of a far-reaching character, and renders 

 observation correspondingly difficult. This peculiarity is that the 

 eggs are enclosed in large numbers in a common capsule, and that of 

 this number only one is destined to develop into an embryo, whereas 

 the rest, termed " vitelligenous," i.e. yolk-bearing cells, are destined 

 to become its food. 



Now, as a result of this arrangement, the earlier stages of 

 development are modified out of all recognition. The first -formed 

 blastomeres are stated to separate from each other completely, then 

 to wander to the periphery of the vitelligenous cells, which they 

 surround, and then to join together to form an embryo. In fact 



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