Hexapoda

[Ken Kinman]

Vladimir Kosel wrote:
     Ostracods have only two pairs of thoracopods and reduced abdomen without segments.
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Dear All,
      Some ostracod workers do not count the 5th pair of limbs as thoracopods, perhaps because it is a walking leg in only some ostracods.  It is thus sometimes referred to as a maxilliped.  The 7th pair of limbs (what is sometimes called the 3rd thoracopod) is also lost in some groups.  There has also long been a lot of uncertainty and disagreement over the homology of various segments.  I believe we must keep an open mind and expect the unexpected, much as theropod workers did when they proposed a developmental "frame shift" to explain the seemingly contradictory digit homologies in the hands of birds and theropod dinosaurs.
      As for the "reduced" abdomen of ostracods, I believe that it is actually the primitive condition for arthropods, and that other arthropods have simply added segments to the thoracoabdomen.  I do not believe Remipedia represents a primitive type of crustacean at all, even though this made sense back when they were thought to have evolved from a  polychaete-like animal with many segments.  I think it is a mistake to limit ourselves to such "many-segmented" crustaceans as the only models for the primitive state.
              ------- Cheers,
                          Ken Kinman