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With the Arthropods,
the progression of protostome
animal groups hits its end. Now we go back to the
deuterostome
line of
animals, which only has two major groups on
it: our group, the chordates, which well get to
soon, and the echinoderms, the starfish, sea urchins,
and their relatives, a rather odd group to be our nearest relatives.
The echinoderms have a number
of odd features:
They show a five-fold circular
pattern called
pentaradial symmetry: in theory,
any one-fifth "pie slice" of an echinoderm should have all
of the same structures as any other. It is likely that
echinoderms originated as a bilaterally
symmetrical group, since their early embryo development
has that layout, but they apparently became sessile, which
seems to lead to radial symmetry, and retained
that pattern even in the descendants who again became crawling browsers,
although its odd that crawling animals didnt tend to also evolve head
ends and bilateral symmetry (only one echinoderm subgroup has).
Since
they have no head end, echinoderms lack central processors or
"brains," although processing
gets done on a
more
spread-out system.
They have a unique powering
system that depends upon
pumping of water into specialized
structures. This
water vascular system exerts a
lot of what force an echinoderm can exert. They do have
muscles, which are important for control, but they do a lot with
tube
feet,
extendable tubes with suction-cup ends that are used
for
crawling and holding onto things.
They
usually have an endoskeleton
that winds up acting more like a shell. Technically,
the skeleton, of calcium salts and proteins (like ours), is inside,
but it is usually covered only by a very thin skin. Even the
spines that give the group its name (echinoderm means "spiny
skin") are really projections of the
skeleton.
Echinoderms are an entirely marine
group, with no evidence of there ever being any fresh water
or land forms even though many species live in the variable
environments of tidal
pools. |