BILATERIA
Animals With Bilateral Symmetry
[© David R]
Chapter Outline
- Description of Bilateria
- Classification of Bilateria
- Xenacoelomorpha
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In the previous two chapters, the animals had body plans with either no symmetry or radial symmetry. Sponges and Placozoans had no symmetry, meaning their shapes were fairly random. Cnidarians and Ctenophorans had radial symmetry. If you placed a plane from the middle of their bodies extending outward, they would have multiple sections that all look alike.
The remaining animals, starting with this chapter, have another kind of symmetry called bilateral symmetry. This means if you placed a plane through the middle of their bodies in a specific way, each side of the plane would mirror the other. Even the Echinoderms in chapter 31, which appear to have radial symmetry as adults, actually have bilateral symmetry as larvae.All phyla within Bilateria, except one, fit into one of two categories, Protostomia or Deuterostomia. Those will be discussed in the following chapters. This chapter will focus on the one phylum that is neither a protostome nor a deuterostome, Xenacoelomorpha.
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