Bilateral symmetry
From Freepedia
Bilateral symmetry is another term for reflection symmetry.
In biology, bilateral symmetry is a characteristic of multicellular organisms, particularly animals. A bilaterally symmetric, or bilaterian, organism is one that is symmetric about a plane running from its frontal end to its caudal end (head to tail), and has nearly identical right and left halves.
Most animals are bilaterally symmetric, including humans (see also facial symmetry). The exceptions are the porifera (sponges) (no symmetry); and the choanoflagellata, placozoa, cnidaria (jellyfish) and the ctenophores, which exhibit radial symmetry. Bilateral animals belongs to a group called Bilateria. The oldest known bilateral animal is called Vernanimalcula.
Other animals which appear to possess radial symmetry have a body plan which is basically bilaterally symmetric. The echinoderms exhibit pentaradial symmetry or pentamerism but this is a secondary characteristic - echinoderm larvae are bilaterally symmetric, as are echinoderm ancestors. Some sea urchins (class Echinoidea) have further modified their radial symmetry to become bilaterally symmetric again.
Adult flatfish have both eyes on the same side of their body, but their body plan is still bilaterally symmetric, but a little twisted.
In biology biradial symmetry means bilateral symmetry with respect to two perpendicular reflection planes, symmetry group C2v.
