Write briefly about symmetry in animals.

Explanation

Symmetry. It means arrangement of different organs in regular manner.
1. Asymmetrical. The animal is called asymmetrical when it cannot be divided into two .equal halves eg Bath sponges, snails.
2. Symmetrical. An animal is symmetrical if it can be divided into equal hialves. The symmetry of the animals can be any one of the following:
(a) Spherical symmetry. The animal when spherical in shape, it can be divided into two equal halves by any cut passing through the centre. The symmetry is said to be spherical eg Sea urchins, Cake urchin etc.
(b) Radial symmetry. Radial symmetry is found. in the animals which have cylindrical shape. The animls can be divided into two equal halves by any vertical cut passing through the centre along the oral-aboral axis. Examples of radially symmetrical animals are - Sycon, Hydra, sea anemone etc.
(c) Pentaradial symmetry. This type of symmetry occurs in starfish, Brittle star etc. The animal having pentaradial symmetry can be divided into equal halves along any of the five radii passing through the middle of any one of the five arms.
(d) Bilateral symmetry. An animal is called biaterally symmetrical when it can be divided into two equal halves only by a single possible median cut passing longitudinally from one end to the other. The two halves are the mirror images of each other. A bilaterally symmetrical animal has an anterior end bearing the mouth and a posterior end bearing the anus. There is a distinct dorsal and a ventral side.
Bilateral symmetry is found in annelids, arthropods, several molluscs and vertebrates.