The Platyhelminthes (Flatworms)

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The Platyhelminthes(Flatworms)
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Major Characteristics

• Animal Characteristics
• Flattened dorsoventrally
• Bilateral Symmetry
• Triploblastic
• Acoelomate
• Incomplete Gut

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Body Plan External

• Bilateral symmetry divided these organisms into
  distinct dorsal, ventral, anterior, and posterior
  portions
• Such symmetry has lead to cephalization in which
  the planaria have specialized senses in the
  anterior portion of their bodies

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Body Plan Internal

• Platyhelminthes are composed of an ectoderm,
  mesoderm, endoderm making them triploblastic
• They contain no cavity or coelome and are thus
  acoelomate

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Movement

• Planaria move using cilia and muscular
  undulations through a mucus that aids in the
  adhesion and traction of the cilia
• Many flatworms are parasitic and dont move

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Feeding

• Unlike most bilateral animals the mouth of the
  planaria is located ventrally and is incomplete
  (without an anus)
• They feed through a retractable pharynx and
  digestion occurs in a distributed digestive
  cavity

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Reproduction

• Asexual reproduction occurs by transverse fission
• Sexual flatworms are monoecious producing both
  sperm and eggs
• Fertilization occurs internally and the offspring
  develops externally and proceeds through a larval
  stage