Origins of Metamerism

A.  What is metamerism
     1.  Serial repetition of body parts.  This can also be a bi-          
          lateral symmetry of body parts, eg., right and left 
          appendages, right and left excretory organs.
     2.  It is not simply the external appearance of segments.
          a.  Flatworms have segments, but are not metameric,
               segments are like individuals in in a colonial
               structure.
          b.  Some nematodes look like they are segmented, but
              there is no internal repetition of parts.
     3.   Metamerism is not always readily apparent from the outside
          a.  Leeches are metameric, but the number of external
               segments is less than the internal segments.
          b.  Humans don't show external segmentation, but arrange-
               ment of spinal nerves indicates metameric nature
               of creature.
B.  How did metamerism come about?
     1.  Some thoughts believe that it is the result of repeated
          asexual reproduction of simple flatworm-like animal
          where the budded-off animals retained their attachments
     2.  Some think it could be incomplete asexual reproduction
C.  What are the advantages of metamerism
     1.  Redundancy-repetition of parts means that if the organs 
          in one segment fail, there are other segments that  may
          still be functional
     2.  Reproduction-Loss of segments containing reproductive cells
          does not destroy the animal--example of the palolo worm of
          the South Pacific.  Worms all rise to the surface at the 
          same time (moon and phase of tide).  Also use of coelomic
          space in segments allow organism to produce sex cells and
          not necessarily impede locomotory abilities
     3.  Increase of size to prevent predation and to facilitate
          predation
     4.  May reason was the enhancement of locomotory abilities.
          a.  Regionalization of contraction-phasic wave of 
               contract-relaxation moves down the length of the
               body.  Enables each segment to work independent of
               the other, so only part of a worm might be using
               metabolic energy for work.  Rest of specimen can
               anchor itself in burrow, etc.  
          b.  Adoption of metamerism may have enabled organism to
               enter substrate rather than remain on surface of 
               substrate.  This provided them with a whole new
               realm for existence.  Burrowing would have provide      
               protection from predators and at the same time new
               resources for the burrower.