THE TRIPLOBLASTIC, ACOELOMATE BODY PLAN

Bilateria

acoelomates


PHYLUM PLATYHELMINTHES - flatworms; 20,000 spp.

Learn about flatworms from the University of California Museum of Paleontology website
Learn about flatworms from the University of Michigan Museum of Zoology website

Class Turbellaria - planarians [fig. 19.1]

  • Characteristics
    1. free living, mainly marine, but freshwater and terrestrial
    2. locomotion via cilia [Fig. 19.3] and  muscular contractions for swimming
    3. gastrovascular cavity  [Fig. 19.4]
    4. pharynx [Fig. 19.5]
    5. protonephridia with flame cells  [Fig. 19.6].  Protonephridial system in a freshwater turbellarian at top right.  image courtesy of BIODIDAC
    6. paired nerve cords (ladder) [Fig. 19.7].  Nervous system of Dugesia at bottom right.   image courtesy of BIODIDAC
    7. cephalization
    8. monoecious [Fig. 19.9]; reproduce both sexually and asexually [Fig. 19.8]

 



Class Monogena [Fig. 19.10]
mainly external parasites of fish; no intermediate host (direct development)
picture and information from Ohio State

Class Trematoda - flukes


Class Cestoda - tapeworms [fig. 19.17]



PHYLUM NEMERTEA - proboscis worms - 900 species

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