Symmetry in animals

Introduction

• 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.

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.

Image source: quora

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.

References: 

i) https://study.com/learn/lesson/symmetry-animals-types-importance.html

ii) https://www.studyandscore.com/studymaterial-detail/symmetry-in-animals-types-of-symmetry-bilateria-and-radiata

Symmetry in animals