<h2>Bilateria</h2>

<p>
The bilateria /ˌbaɪləˈtɪəriə/ are the animals with bilateral symmetry, i.e. they have a front and a back end, as 
well as an upside and downside, and therefore a left and a right. In contrast, radially symmetrical animals like 
jellyfish have a topside and downside, but no front and back.
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<p>
The bilateria are a major group of animals, including the majority of phyla but not sponges, cnidarians, placozoans and 
ctenophores. For the most part, bilateral embryos are triploblastic, having three germ layers: endoderm, mesoderm, and ectoderm. 
Nearly all are bilaterally symmetrical, or approximately so; the most notable exception is the echinoderms, which 
achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae.
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<p>
Except for a few phyla (i.e. flatworms and gnathostomulids), bilaterians have complete digestive tracts with a separate mouth and anus. 
Some bilaterians lack body cavities (acoelomates, i.e. Platyhelminthes, Gastrotricha and Gnathostomulida), while others display primary 
body cavities (deriving from the blastocoel, as pseudocoel) and/or secondary cavities (that appear de novo, for example the coelom).
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