Symmetry in animals

  • Symmetry means the arrangement of body parts into geometrical designs.
  • Many adult sponges that are irregular in shape are said to be asymmetrical.
  • However, the sponges start their life from a radially symmetrical larva.
  • Four types of symmetry are found in animals. They are: spherical symmetry, radial symmetry, bi-radial symmetry and bilateral symmetry.

Spherical symmetry

  • Any plane passing through the centre divides the body into equivalent or mirrored half is the spherical symmetry.
  • Spherical forms are best suited for floating and rolling.
  • Spherical symmetry is found in protozoa (e.g., Volvox, Heliozoa, Radiolaria) and is rare in animals.

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Radial symmetry

  • In this type of symmetry, body can be divided into similar halves by more than two planes passing through one main axis.
  • Radial animals are usually sessile, freely floating or weakly swimming.
  • Radial symmetry is found in some sponges, coelenterates and echinoderms.

Bi-radial symmetry

  • In this symmetry, only two planes passing through the longitudinal axis will produce mirrored halves.
  • Bi-radial symmetry is a variant form of radial symmetry which is found in sea walnuts (phylum: Cnetophora) and sea anemones (Anthozoa).
  • It is also taken as the combination of radial and bilateral symmetry.

Bilateral symmetry

  • This symmetry is also called plane symmetry.
  • Bilateral animals are collectively called the Bilateria.
  • Animals that can be divided along a median longitudinal or sagittal plane into two halves are called Bilateral symmetry.
  • Bilateral symmetry is strongly associated with cephalization, which is the modification of anterior or oral end of the animal into definite head.
  • About 99% of animals are bilaterally symmetric including humans.

Symmetry in animals

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