(By Dr. Girish Chandra)
Arrangement of body parts in a balanced geometrical design, divisible into equal parts by lines or planes of division is called symmetry. Asymmetrical animals such as Amoeba or sponges possess irregular body shapes and hence have no symmetry but higher metazoans possess some kind of symmetry depending on their habits and habitats that balances their activities. Symmetry can be of the following types:
1. Spherical symmetry. As found in Volvox, Actinophrys (Heliozoa) and Thalassicola (Radiolaria), the body is ball-like and all planes passing through the centre of body will cut the animal into equal halves. This type of symmetry is useful in rolling movement, for floating in water or in sedentary habits in which case food is available in all directions. Body organs like cilia or tentacles are located all around the body in a radiating manner.
2. Radial symmetry. This type of symmetry is found in coelenterates and echinoderms in which Body parts (antemeres) are arranged along the main longitudinal axis of body. It is best suited for sessile existence where food is planktonic and available in abundance in all directions. Food capturing organs are therefore arranged radially and the animal does not have to move in search of food that is floating all around the body. Some of the echinoderms, like star fishes, later gave up their sessile existence to become hunters in pursuit of larger prey but they could not give up their ancestral radial symmetry, although bilateral symmetry is more suited for active carnivorous habit.
3. Biradial symmetry. The term biradial symmetry denotes a mixture of bilateral and radial symmetry in these animals. This is found in Ctenophores (Acnidaria, which are also called comb-jellies) which are not sedentary but floating animals and show a mixture of bilateral and radial symmetries. Animals such as Pleurobrachia have oval body on which eight comb plates are radially arranged like bands and are used for swimming, whereas mouth, anal pore and statocysts are placed on the anterio-posterior axis. But they also have a pair of retractile tentacles that bear colloblasts (lasso cells) which secrete sticky substance that helps in capturing planktonic food on which they feed. Tentacles demonstrate bilateral symmetry whereas comb plates show radial symmetry and the animal takes advantage of both symmetries for food hunting and active swimming.
4. Bilateral symmetry. This type of symmetry is found in most of the higher animals above Platyhelminthes and is best suited in animals which move in a definite direction, due to which the sense organs and nervous system concentrate on the anterior side and locomotory organs become paired for balanced propulsion of body. A single line passing through the longitudinal axis will divide the body into two equal halves in such a way that one half is a mirror image of the other. Flat worms were the first bilaterally symmetrical animals and other higher groups such as annelids, arthropods, some molluscs and chordates are all bilaterally symmetrical.