Everything about Bilateria totally explained
The
Bilateria are all animals having a
bilateral symmetry, for example they've a front and a back end, as well as an upside and downside.
Radially symmetrical animals like
jellyfish have a topside and downside, but no front and back. The bilateralians are a
subregnum (a major group) of
animals, including the majority of
phyla; the most notable exceptions are the
sponges and
cnidarians. For the most part, Bilateria have bodies that develop from three different
germ layers, called the endoderm, mesoderm, and ectoderm. From this they're called
triploblastic. Nearly all are bilaterally symmetrical, or approximately so. The most notable exception is the
echinoderms, which are radially symmetrical as adults, but are bilaterally symmetrical as larvae.
Except for a few highly reduced forms, the Bilateria have complete
digestive tracts with separate
mouth and
anus. Most Bilateria also have a type of internal body cavity, called a
coelom. It was previously thought that acoelomates gave rise to the other group, but there's some evidence now that in the main acoelomate phyla (
flatworms and
gastrotrichs) the absence could be secondary. The indirect evidence for the primitivity of the coelom is that the oldest known bilaterian animal,
Vernanimalcula, had a structure that could be interpreted as a body cavity.
There are two or more
superphyla (main lineages) of Bilateria. The
deuterostomes include the
echinoderms,
hemichordates,
chordates, and possibly a few smaller phyla. The
protostomes include most of the rest, such as
arthropods,
annelids,
mollusks,
flatworms, and so forth. There are a number of differences, most notably in how the
embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes. Many
taxonomists now recognize at least two more superphyla among the protostomes,
Ecdysozoa (molting animals) and
Lophotrochozoa. Some taxonomists also recognize another protostome superphylum,
Platyzoa, while others would include the Platyzoans in Lophotrochozoa or not include them in any superphylum. The arrow worms (
Chaetognatha) have proven particularly difficult to classify, with some taxonomists placing them among the deuterostomes and others placing them among the protostomes. The two most recent studies to address the question of chaetognath origins (Helfenbein
et al. 2004 and Papillon
et al. 2004) support protostome affinities.
The hypothetical last common ancestor of all bilaterians has sometimes been called
Urbilateria. Since the name refers to the concept of the ancestral bilaterian, it doesn't refer to any animal fossil, nor is it a taxonomically valid genus designation. Other terms that can be used are
Last common bilaterian ancestor and
Protostome-Deuterostome ancestor. The fossil
Vernanimalcula may be the earliest known bilaterian.
Phylogeny
A proposed phylogeny of the Bilateria after Nielsen is as follows. An alternate phylogeny suggests a basal group called the
ecdysozoa.
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Bryozoa
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|label2=Cycloneuralia
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|label3=Cephalorhyncha
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|label2=Deuterostomia
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Further Information
Get more info on 'Bilateria'.
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