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.