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Each new group seems to bring some huge evolutionary leap
that will form a new foundation for later groups to build upon, and this
group is no exception. The new feature this time shows up in a few
mollusks, such as the chitons, but its unclear what effect it had on most
of the mollusks history. There seems a good chance that this
chapters group has some mollusks
on its family tree.
The feature is segmentation (technically
called
metamerism, probably to avoid confusion among all the
different types of "segments" in
animals). The basic approach is to come up with a basic
body "compartment," like cars on a train, and build up a body by
adding more. It is a quick and simple way to add size, and segmented
worms, also known as annelids, are the
prototypes. In animals that stay fully segmented, it helps make
movement more efficient, but in many groups it shows up mostly as a developmental pattern - our phylum, the Chordates, are considered
segmented, although the patterns are clearer in embryos than in adults.
Somewhat like a classic train with its engine and
caboose, segmented animals use their repeating segments between
specialized front and back compartments. There is a typical head,
with a mouth and sensory organs and processors (segmented worms show cephalization
and the connected bilateral
symmetry), and a back end,
with the exit of the digestive system and sensory organs to detect things
sneaking up from behind. The digestive system usually is not
segmented, just running down the sequence of segments in its classic
disassembly-line configuration.
A typical segment
has similar set-ups in blood vessels (segmented worms circulate their
blood in tubes, a system called closed circulation);
nerves (each segment connects to the next along the
typical invertebrate
central nervous system, two solid cords running along the ventral
[belly] side), usually with processing ganglia in each
segment; muscles, running both lengthwise (longitudinal)
and around (circular); a doughnut-shaped, fluid filled
chamber that acts as a
hydrostatic
skeleton; excretory
structures; and hard bristles called setae sticking
out and often supporting paddlelike structures called
parapods.
This sequential pattern of repeating
architecture is called serial homology, and is the reason
why human arms and humans have the same basic bone structure.
Segmentation is a very efficient way to build an animal,
but being extremely segmented once the pattern is set is apparently
unnecessary. Even in the segmented worms, there are whole groups
where the segmentation in adults is mostly on the surface. Some
segmented animals in other groups, like the centipedes, remain very much
segmented throughout their lives; others, like flies or humans, use
a segmented pattern to lay out their embryos and then modify it until it
almost disappears.
Some sources claim that the segmented muscle layout and articulations
between each segment lead to more efficient movement, and that may have
been part of the advantage as segmentation first evolved. It must be
a minor advantage, however, since many efficiently-moving animals are
among those that largely lose their segmentation during development.
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