Academic Script. Symmetry. 1 st Year Undergraduate Model 1 UGC Syllabus. Animal Diversity - I. Topic No. & Title: Topic : A-3 Symmetry

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1 Academic Script Symmetry Course Name: Paper No. & Title: Zoology 1 st Year Undergraduate Model 1 UGC Syllabus Z 101 B Animal Diversity - I Topic No. & Title: Topic : A-3 Symmetry

2 Academic Script: 1.Introduction Symmetry means an arrangement of body parts into geometrical designs. It refers to the division of body into equal parts by lines or planes. Symmetry in biology is the balanced distribution of duplicate body parts or shapes. Symmetry is important in the taxonomy of animals. Current Zoological ideas about symmetry derive chiefly from Haeckel. An animal is called symmetrical when a plane passing through its centre will divide it into similar halves. When an animal cannot be divided into like parts by a plane, it is called asymmetrical. All animals are either asymmetrical or symmetrical. Examples of asymmetrical animals are sponges, some Protozoa (e.g. Amoeba) and few others. Certain terms are often used for explaining symmetry. An axis is an imaginary line passing through the centre of body, such as longitudinal axis and oral-aboral axis. Either end of an axis is termed a pole. Thus, each axis has two poles. A plane of symmetry is a straight line that divides organisms into corresponding halves. Metazoa display only two types of symmetry (radial and bilateral). Two other types of symmetry are also recognized (Spherical and Biradial). Protozoa are not

3 only asymmetrical but display all four types of symmetry in their diverse body forms. 2.Evolution of Symmetry Almost all animals exhibit some form of symmetry; the main exceptions are sponges, which are asymmetrical in shape and Cnidarians exhibit radial symmetry. The remainders of the phyla listed below have bilateral symmetry (Like Flatworms, Mollusca, Annelids, Roundworms, Arthropod, Echinoderms and Chordates). Bilateral symmetry is important in the evolution of animals because it provides a balanced distribution of equal body parts or shapes that extend from the centre of the oral surface to the end of the body. The most notable exception is the echinoderms, which achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae.

4 Body Symmetry Fundamental aspect of an animal's body organization is its body shape, form and the manner in which body parts are arranged. The word symmetry is used to describe the arrangement of body structures with reference to the body axis. Animals which can be bisected or split along at least one plane so that the resulting halves are similar to each other are described as symmetrical. Animals which cannot be split into similar halves are described as asymmetrical. There are four basic forms of symmetry (like spherical, radial, biradial, bilateral). Forms of Animals Asymmetrical Symmetrical Spherical Radial Biradial Bilateral

5 3.Types of Symmetry (1) Spherical symmetry. It is found in animals whose body has the shape of a sphere. A sphere has an infinite number of planes of symmetry that can pass through the centre to divide it into similar halves. In the strict sense, such symmetry is rare in animals and is exhibited only in some protozoans such as radiolarians. In such animals, there is no polarity as in other symmetrical animals. All plans that pass through the centre will cut it into similar halves. Some protozoans (e.g. Volvox, Heliozoa and Radiolaria) have spherical symmetry, and it is adapted for free-floating or rolling movements. (2) Radial symmetry. The body is in the form of a flat or tall cylinder. Many similar body parts, called antimeres, are arranged around one main, central or longitudinal axis in a circular or radiating manner like the spokes of a wheel. All the lines passing through this longitudinal axis, in any plane, will divide the body into equal halves or antimeres. The surface having mouth is the oral surface, and the opposite surface is the aboral surface. Examples are echinoderms (Starfish, Sand dollar and Sea urchins) and most coelenterates (hydra, coral polyp and jelly fish).

6 The coral polyp has radial symmetry. This was the first type of symmetry to evolve. The coral has a distinct top and bottom but not distinct ends. It can be divided into identical halves like a pie, but not into right and left halves. Animals with radial symmetry have no sense of directions such as forward and backward or left and right. This makes controlled movement in these directions impossible. Radial symmetry is best suited for a sessile existence. Most of them are attached by the aboral surface. Some are free-swimming but remain at the mercy of water currents. Due to similarity of antimeres, their sensory receptors are equally distributed all around the periphery. This enables them to receive stimuli and to meet the challenges of the environment equally from all directions. They can obtain food or repel enemies from all sides.

7 In the animal kingdom, radially symmetrical phyla are Porifera, Coelenterata, Ctenophore and Echinodermata. Out of these, only Coelenterata and Ctenophore display a fundamental radial symmetry. Both the phyla were grouped together by Hatschek ( ) under the division Radiata. Adult Porifera are mostly asymmetrical but they start life from a radially symmetrical larva. On the other hand, larval stage of Echinodermata has bilateral symmetry but the adults become radially or pentaradially symmetrical (Starfish, Brittle star and Sea anemone). (3) Biradial symmetry Biradial is a combination of radial and bilateral symmetry. Ctenophores exhibit biradial symmetry. Some of the phyla formerly

8 classified as aschelminth have anterior and radial portions. It is a variant form of radial symmetry found in Ctenophora and most Anthozoans of Coelentrates (e.g. anemones), and is the best fitted for a floating life. Such symmetry has only two planes of symmetry, one through the longitudinal and sagittal axises, and transverse axises, which will divide the animal into equal halves. (4) Bilateral symmetry In most of the higher animals, the longitudinal axis of body runs from the anterior end (head) to the posterior end (tail). There is a single plane, the median longitudinal or saggittal plane, through which the body can be divided into two similar right and left halves. This is called bilateral symmetry. Besides right and left sides, an upper or dorsal surface and a lower or ventral surface are also recognizable, which are unlike because they are exposed to different conditions.

9 Bilateral symmetry is characteristic of the most successful and higher animals, including the remaining invertebrates and all vertebrates. In most of them, the anterior end is differentiated into a head. A bilateral body plan makes movement easier, which helps animals to avoid predators and hunt for food. It may also favor a central nervous system, such as our spinal cord with its network of nerves leading up to the brain. Some studies suggest that animals look for symmetry when they are choosing a mate. First phylum of animal kingdom to exhibit bilateral symmetry is the Plathelminthes. All bilaterally symmetrical metazoans were grouped together by Hatschek under the Division Bilateria. Such as echinoderms, display radial symmetry which have been secondarily derived from bilateral ancestors due to assumption of an attached mode of life by adults. Examples of bilateral symmetry (Humans, cats, dogs, butterflies etc.). 4.Summary Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit

10 some form of symmetry, either radial symmetry or bilateral symmetry or spherical symmetry. A small minority exhibits no symmetry (are asymmetric). All vertebrates and many invertebrates have these kinds of symmetry. Radial symmetry and its derivatives (like biradial e.g. sea anemone and penta radial e.g. starfish) appear to be primitive features of some animals, as they restrict their movements. In bilateral symmetry (also called plane symmetry), only one plane, called the sagittal plane, will divide an organism into roughly mirror image (e.g. butterfly, human, dog, cat etc.)