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Symmetry in Animals – Definition, Types and Importance

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  • Last Updated : 04 Aug, 2022
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Different animals with different structures and forms are present all over the world. As over million species of animals have been described till now need for classification becomes all the most important. Classification helps in the easy study of species and gives a systematic position to newly described species. The basic classification includes Levels of organization, Symmetry, Diploblastic and Triploblastic Organisation, Coelom, Segmentation, and Notochord. Hence, all these categories must be kept in mind while classifying an animal. 

Symmetry

Symmetry in biology is the balanced arrangement of body parts or shapes around a central point or axis. That is, the size, shape, and relative location on one side of a dividing line mirrors the size, shape, and relative location on the other side. Animals can be categorized on the basis of their symmetry. The repetition of the parts in an animal or plant in an orderly fashion. Specifically, symmetry refers to the correspondence of body parts, in size, shape, and relative position, on opposite sides of a dividing line or distributed around a central point or axis. Symmetry means to the dividing line which separates the body of an organism into parts asymmetrically, radially, and bilaterally. It has importance in its own classification. Various animals show symmetry in different ways such as the Sycon of sponges shows Radial symmetry. Like sycon many other animals show symmetry differently. Hereunder, are the types of Symmetry

Types of Symmetry

 

  • Spherical Symmetry. The body has the shape of a sphere and the parts are arranged concentrically around or radiate from the center of the sphere. Such an animal has no ends or sides, and any plane. It is found mainly in Protozoa(e.g., Volvox, Heliozoa, and Radiolaria). It is rare in animals. Spherical symmetry is also found in eggs and early embryos of some animals.
  • Radial Symmetry. Radial symmetry is a symmetry in which the sides exhibit correspondence or regularity of parts around a central axis. It is lacking the left and right sides. It is in contrast to bilateral symmetry which is more common than radial symmetry. It is found in some sponges(Sycon), Coelenterates(e.g., Hydra, jellyfish),echinoderms(e.g.,starfish).
  • Biradial Symmetry. Biradial symmetry is found in organisms that show morphological features (internal or external) of both bilateral and radial symmetry. e.g, sea anemones. The animals which show radial and biradial symmetry have oral and aboral sides. The oral side is that which has a mouth, whereas the aboral side is one that is opposite to the oral side.
  • Bilateral Symmetry. Symmetry in which similar anatomical parts are arranged on opposite sides of a median axis so that only one plane can divide the individual into essentially identical halves. Bilateral symmetry is found in many invertebrates, e.g., annelids arthropods, etc., and all vertebrates. Bilateral symmetry is strongly associated with cephalization.

Importance of Symmetry

  • The equal distribution of body parts and sense organs makes them better able to react to environmental stimuli coming from all around their bodies. 
  • As the shapes of organs and cells are strictly connected to their activities and functions, symmetry is an important matter also at those scales.
  • Symmetry creates balance, and balance in design creates harmony, order, and aesthetically pleasing results.

Symmetry in Plants

Plants exhibit both radial symmetry and bilateral symmetry, often at the same time. In flowers, it has a lot to do with pollination strategy. A sunflower head has radial symmetry, allowing small insects to land on top, while a snapdragon flower has bilateral symmetry, inviting large bees to enter from the side. In biology, symmetry is approximate. For example, plant leaves, while considered symmetric, will rarely match up exactly when folded in half. Furthermore, symmetry may refer only to the external form and not the internal anatomy.

Frequently Asked Questions

Question 1: What is Symmetry?

Answer:

 Symmetry refers to the correspondence of body parts, in size, shape, and relative position, on opposite sides of a dividing line or distributed around a central point or axis.

Question 2: What are the types of symmetry?

Answer:

Spherical, Radial, Biradial, and Bilateral Symmetry are the types of Symmetry.

Question 3: Give an example of Bilateral Symmetry.

Answer:

Annelids, Arthropods, etc are examples of Bilateral Symmetry.

Question 4: What is the Importance of Symmetry?

Answer:

The equal distribution of body parts and sense organs makes them better able to react to environmental stimuli coming from all around their bodies. 

Question 5: Define Radial Symmetry.

Answer:

It is found in some sponges(Sycon), Coelenterates(e.g., Hydra, jelly fish),echinoderms(e.g.,star fish).

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