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It used to be thought that all of our
close relatives would be vertebrates,
with backbones. But a couple of small sea creatures without
backbones still shared a number of distinct features with vertebrates,
indicating that they were close relatives, so the phylum was expanded and now
includes a couple of types of invertebrates in the
groups; the vertebrates, previously a phylum-level group, are
now a sub-phylum inside the phylum of chordates.
Here
is a
picture showing the features that the chordates share. We
are chordates, but don't go looking for all of the traits on
your person - many of the features were only present during your
embryo days. This is part of what led to the idea of "ontogeny
recapitulates phylogeny," since we do see several
"ancestral" traits only in developing embryos.
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Chordates get their names from a structure
called the notochord. A notochord has two
different uses: in primitive chordates, it's a flexible
skeletal rod useful for the motion of swimming; in
vertebrates, it appears very early in
embryos and releases chemicals
that
cause the layers of the ectoderm over it to
thicken and roll up, becoming the spinal cord and
usually a brain. Vertebrate notochords break
down and are absorbed after they have done that single job.
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Chordates have a
nervous system based
upon a single, hollow nerve cord located under the dorsal
(back) surface. Remember, invertebrates
- the mollusks, the segmented worms, the arthropods - have double,
solid, ventral nerve cords.
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Chordates have
pharyngeal
(throat) pouches with
arches of arteries
in them and gill slits to the outside. Many
chordates develop gills from at least some of these structures, but
land vertebrates, although they have them as embryos, modify them
into things such as the roof of the mouth or the voice box. If
your embryology mutated and you were born with functioning gills
(which couldnt really happen), you'd almost certainly be missing a
bunch of important throat structures - and you wouldn't be able to
use the gills anyway, since water doesn't hold enough oxygen to
sustain a human.
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Chordates, like many animals, typically have
tails, but only chordates have tails behind their anus.
Sometimes these characteristic features are a bit odd.
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There are several chordate features that have already shown up in
other animal groups, although there isn't necessarily any
indications that the other animals are closely related to us. Chordates,
like segmented worms, have a circulation system with blood vessels
everywhere, a closed
circulation system - although the systems are
"flipped," with ventral (belly-side) hearts
in chordates and dorsal hearts in the worms. Also, chordates
are basically segmented (metameric) animals, although
this repeating pattern often can be seen strongly only as embryos
develop. You can see a hint of segmentation in the
construction of the backbone, its attached muscles, and our peripheral nervous systems,
though. Chordates also commonly have
endoskeletons. |