Symmetry in biology

Symmetry in biology is the balanced distribution of duplicate body parts or shapes within the body of an organism. In nature and biology, symmetry is always approximate. For example, plant leaves – while considered symmetrical – rarely match up exactly when folded in half. Symmetry creates a class of patterns in nature, where the near-repetition of the pattern element is by reflection or rotation. Symmetry in biology is the balanced distribution of duplicate body parts or shapes within the body of an organism. In nature and biology, symmetry is always approximate. For example, plant leaves – while considered symmetrical – rarely match up exactly when folded in half. Symmetry creates a class of patterns in nature, where the near-repetition of the pattern element is by reflection or rotation. The body plans of most multicellular organisms exhibit some form of symmetry, whether radial, bilateral, or spherical. A small minority, notably among the sponges, exhibit no symmetry (i.e., are asymmetric). Symmetry was once important in animal taxonomy; the Radiata, animals with radial symmetry, formed one of the four branches of Georges Cuvier's classification of the animal kingdom. Radially symmetric organisms resemble a pie where several cutting planes produce roughly identical pieces. Such an organism exhibits no left or right sides. They have a top and a bottom surface, or a front and a back. Symmetry has been important historically in the taxonomy of animals; Georges Cuvier classified animals with radial symmetry in the taxon Radiata (Zoophytes), which is now generally accepted to be a polyphyletic assemblage of different phyla of the Animal kingdom. Most radially symmetric animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth, to the center of the opposite, aboral, end. Radial symmetry is especially suitable for sessile animals such as the sea anemone, floating animals such as jellyfish, and slow moving organisms such as starfish. Animals in the phyla Cnidaria and Echinodermata are radially symmetric, although many sea anemones and some corals have bilateral symmetry defined by a single structure, the siphonoglyph. Many flowers are radially symmetric or actinomorphic. Roughly identical flower parts – petals, sepals, and stamens – occur at regular intervals around the axis of the flower, which is often the female part, with the carpel, style and stigma. Many viruses have radial symmetries, their coats being composed of a relatively small number of protein molecules arranged in a regular pattern to form polyhedrons, spheres, or ovoids. Most are icosahedrons. Tetramerism is a variant of radial symmetry found in jellyfish, which have four canals in an otherwise radial body plan. Pentamerism (also called pentaradial and pentagonal symmetry) means the organism is in five parts around a central axis, 72° apart. Among animals, only the echinoderms such as sea stars, sea urchins, and sea lilies are pentamerous as adults, with five arms arranged around the mouth. Being bilaterian animals, however, they initially develop with mirror symmetry as larvae, then gain pentaradial symmetry later. Flowering plants show fivefold symmetry in many flowers and in various fruits. This is well seen in the arrangement of the five carpels (the botanical fruits containing the seeds) in an apple cut transversely.