Two types of bilateral symmetry in the Metazoa: chordate and bilaterian.

@article{Jefferies1991TwoTO,
  title={Two types of bilateral symmetry in the Metazoa: chordate and bilaterian.},
  author={Richard P. S. Jefferies},
  journal={Ciba Foundation symposium},
  year={1991},
  volume={162},
  pages={
          94-120; discussion 121-7
        }
}
The chordate sagittal plane is perpendicular to the sagittal plane primitive for the bilaterally symmetrical metazoans (Bilateria). The earliest metazoans, when symmetrical at all, were probably radial in symmetry. The axis of symmetry was vertical and the mouth, when present, opened either upward or downward. The Bilateria evolved from the primitive metazoan condition by acquiring bilateral symmetry, mesoderm, a brain at the anterior end and protonephridia. Perhaps in the stem lineage of the… 

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The Scottish specimens described in this memoir are all the property of Mrs Robert Gray, and I am exceedingly indebted to her, not only for entrusting them to me for description, but for allowing