Types of Symmetry

Spherical  Symmetry :-

Spherical symmetry is also called Homaxial apolar symmetry. Spherically symmetrical forms are best suited for floating and rolling. The rare type of symmetry is spherical symmetry. Spherical symmetry is found in Heliozoans and radiolarians. Body of the spherically symmetrical animal can be cut into two identical halves in any one of the infinite number of planes that pass through the central point. Body parts are arranged concentrically around or radiating from a central point in spherical symmetry.

Radial  Symmetry :- 

Radial symmetry is also called monaxial heteropolar symmetry. In radial symmetry the sensory and feeding structures are uniformly distributed around the body hence they can interact with their environment equally in all the directions. In radially symmetrical animals body is cylindrical (ex- Hydra), or vase- like ( sponges) or umbrella shaped (ex- Jelly fish). In Radially symmetrical animals the principal axis is oral-aboral axis it is heteropolar. Anterior, posterior ends and dorsal, ventral, lateral surfaces are not differentiated in radially symmetrical forms as well as spherically symmetrical forms. Radial symmetry is found in some sponges and most of the Cniderians. Echinoderms are primarily bilateral animals (their larvae are bilateral). Most of the echinoderms have secondarily become radial, exhibiting pentamerous radial symmetry in the adult stage.

Biradial  Symmetry :- 

The Biradial type of symmetry of sea anemone seems to have been derived from the radial type, which is primarily by the elongation of the mouth and associated parts. Biradial symmetry has two planes of symmetry. Ctenophores and most anthozoans exhibit biradial symmetry. In ctenophores most of the body parts radially arranged but the tentacles are paired. A sea anemone, with two siphonoglyphs does exhibit biradial symmetry. While a sea anemone with one siphonoglyph exhibit radial symmetry.

Bilateral Symmetry :- 

In Bilateral symmetry the principal axis is the anterior- posterior axis, it is heteropolar, with differentiate anterior and posterior ends. In Bilateral symmetry sagittal axis is heteropolar and the transverse axis is apolar. In bilatearl symmetry there is only one planes of symmetry, it is median sagittal plane. The appearance of bilateral symmetry was a major advancement in animal evolution. Cephalization is associated with bilateral symmetry. As a result of cephalization, bilaterally symmetrical animals can sense the new environment into which they enter and respond to it more efficiently than other animals. All the tiploblastic animals of animal kingdom exhibit bilateral symmetry.

Evolutionary advantages of Bilateral Symmetry :-  

• Here parts of bodies can develop differently; different organs can be located in different body parts.
• Here the movement made more efficient (direction of movement better defined, resistance to water or sediments reduced).
• It brings higher mobility as higher flexibility becomes a key factor for more efficient in seeking food, finding mate, avoiding and escaping predators.