- 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.
- 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.
- 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.
- 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.
- 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.