Bilateria
Protostomia
Deuterostomia
</table>
The
Bilateria is a branch of
metazoa. These
animals have a body composed of three different germ layers, called the
endoderm,
mesoderm, and
ectoderm. These animals are also called
triploblastic.
The Bilateria include all animals that are (more or less) bilaterally symmetrical and, except for a few
flatworms, have an internal
body cavity (
coelom or
pseudocoelom) with a mouth at one end and an anus at the other.
The Bilateria include the majority of animals, with the
cnidarians and
sponges being the principal exceptions.
The two superphyla of the Bilateria are
Deuterostomes (
echinoderms,
chordates,
hemichordates,
Chaetognatha and
Vetulicolia) and
Protostomes (
arthropods,
annelid worms,
molluscs and many more groups). They are distinguished by how the coelom develops, in particular whether the first body opening to form in larval growth becomes the anus (deuterostomes) or mouth (protostomes). Another distinguishing characteristic is how the third set of cells to form after egg fertilization orient themselves during cleavage. Protostomes have spiral cleavage, while deuterstomes have radial cleavage.
