page_title: Symmetry in biology
section_title: Subtypes of radial symmetry
hierarchical_section_title: Symmetry in biology / Radial symmetry / Subtypes of radial symmetry
caption_attribution_description: Nederlands: Sterappel dwarsdoorsnede Français&#160;: Coupe équatoriale d'une pomme Sterappel English: Cross section of a Sterappel apple
context_page_description: Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a plane of symmetry down its centre, or a pine cone with a clear symmetrical spiral pattern. Internal features can also show symmetry, for example the tubes in the human body which are cylindrical and have several planes of symmetry.
  Biological symmetry can be thought of as a balanced distribution of duplicate body parts or shapes within the body of an organism. Importantly, unlike in mathematics, symmetry in biology is always approximate. For example, plant leaves – while considered symmetrical – rarely match up exactly when folded in half. Symmetry is one class of patterns in nature whereby there is near-repetition of the pattern element, either by reflection or rotation.
  While sponges and placozoans represent two groups of animals which don't show any symmetry, the body plans of most multicellular organisms exhibit, and are defined by, some form of symmetry. There are only a few types of symmetry which are possible in body plans.
context_section_description: Some jellyfish, such as Aurelia marginalis, show tetramerism with a four-fold radial symmetry. This is immediately obvious when looking at the jellyfish due to the presence of four gonads, visible through its translucent body. This radial symmetry is  ecologically important in allowing the jellyfish to detect and respond to stimuli (mainly food and danger) from all directions.
  Flowering plants show five-fold symmetry, or pentamerism, in many of their flowers and fruits. This is easily seen through the arrangement of five carpels (seed pockets) in an apple when cut transversely. 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.
  Hexamerism is found in the corals and sea anemones (class Anthozoa) that are divided into two groups based on their symmetry. The most common corals in the subclass Hexacorallia have a hexameric body plan; their polyps have six-fold internal symmetry and a number of tentacles that is a multiple of six.
  Octamerism is found in corals of the subclass Octocorallia. These have polyps with eight tentacles and octameric radial symmetry. The octopus, however, has bilateral symmetry, despite its eight arms.
caption: Apple cut horizontally showing that pentamerism also occurs in fruit
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