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 Platyhelminthes - "the flatworms"
This phylum's 'place'
relative to other phyla:
  • Metazoan
    • Triploblast
      • "Platyhelminthes-Rotifer-Lophotrochozoa" group
Selected taxa within this taxon:
  • Class Trematoda
  • Class Turbellaria
  • Class Cestoda
  • (Class Monogenea)

Platyhelminthes are a diverse group of flatten worms with a fairly simple body plan
18,500 species


Body Plan

  A cautionary note about interpreting body plans that a more simple:

Traditionally considered the most primitive of the bilateria with the bilateria being grouped into acoelomates, psuedocoelomates, and coelomates.

Why were Platyhelminthes considered the primitive outgroup?


Now many zoologist are leaning toward three major branches of the bilateria where platyhelminthes and rotifers are closely related to lophotrochozoa and other acoelomates less related based primarily on molecular similarities.

 The acoelomate condition may be secondary:

  • filled in as an adaptive response to reduce body size
  • neotonous from stage prior to coelom development.   All three conditions may have occurred in the development sequence of the original bilaterian ancestor. 
  • Are lack of systems and lack of coelom indications of primitive conditions or the result of constraints associated with selection for flatten morphology?

    LESSON: Do not assume simplicity is necessarily primitive.

    Could traditional thoughts on phylogeny been influence in part by "world views" of the way we thought nature ought to be?
    For example, the Great Chain of Being, while discounted, has resulted in the idea of 'progress'  being pervasive in modern civilization. 

    However, there appears to be one group that may be the outgroup of the bilateria phyla (based on molecular evidence and supported by some morphological evidence): the Turbellarian order Acoels of the Platyhelminthes.  If the Acoels are "the earliest branch within the bilaterian clade that left an extant should ancestor",  should this group be considered as a separate phylum?



    Natural History


    Classes of Platyhelminthes

    Class Turbellaria



    Platyhelminthes classes as well as the phylum may be paraphyletic.  Fossil records of Platyhelminthes are minimal. 
    Why might they not leave many trace fossils?

    Ideas to consider from the platyhelminthes:
    Why might a group of organisms be "simple"?  Can organisms evolve simplicity in some ways but not others?
    Lecture Sources:
    • Pechenik. Jan A.  2000. Biology of the Invertebrates.  McGraw-Hill, New York.
    • Ruiz-Trillo I, Ruitort M, Littlewood DTJ, Herniou EA, Baguna J. 1999. Acoel flatworms; Earliest extant bilaterian metazoans, not member of platyhelminthes. Science 283: 1919-1923.

    • Rieger RM. 1986. On the origin of the Bilateria: Significance of ultrastructural research for a new understanding of metazoan phylogeny. From http:/

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