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In general terms, symmetry refers to the type of arrangement of body parts according to which if we draw an imaginary line down the middle, both resulting parts would be the same. In most multicellular organisms there is some kind of symmetry. Humans, for example, present a bilateral symmetry in which the distribution balanced by the sagittal plane divides the body into left half and right half.
However, not all organisms exhibit this type of symmetry. Although most organisms have some form of symmetry, this could be bilateral or radial.
radial symmetry
This type of symmetry is defined by an axis that is different at its two ends (heteropolar) and from which symmetry principals are established. When an organism is radially symmetrical, it can be cut from one side through the center to the other side and this cut would produce two equal halves, similar to cutting a birthday cake. Regardless of which end you start the cut from, if you cut from one side to the other through the center, both halves will be the same.
pentaradial symmetry
This type of symmetry is unique to echinoderms, such as starfish and some types of sea urchins. It is a system that starts from the oral cavity of the organism and that makes them have five equal parts.
In the case of sea stars, their pentaradial symmetry is secondary since while they were larvae they presented bilateral symmetry.
biradial symmetry
Also called false bilateral symmetry . It is present in organisms such as ctenophores, which are characterized by producing a sticky substance to capture their prey and are exclusively marine. They present two axes of symmetry and could be divided into four parts along a central plane. Each of these parts would be identical to its opposite side, but not to its adjacent side.
Characteristics of Radially Symmetrical Animals
Radially symmetric animals have an upper part where the mouth is located and a lower part called the aboral side. In this sense, radial animals do not have a front, back, right or left side.
Radially symmetric animals can usually move in any direction. They tend to move slowly or drift with the waves and currents. This is in contrast to bilaterally symmetric organisms that tend to move forward to move or search for food, such as seals, whales, and humans.
Instead of possessing a centralized nervous system, radial organisms have sensory structures scattered throughout their bodies. Starfish, for example, have eyespots at the end of each of their arms instead of being concentrated in the region that would occupy the head.
Examples of radially symmetric marine animals
These are some of the animals that present radial symmetry:
- sea polyps.
- Jellyfish.
- The sponges.
- The sea stars.
- The sea urchins.
References
Baguña et al. (2002). Origin and evolution of body axes and bilateral symmetry in animals. University of Barcelona. Available at: http://repository.essex.ac.uk/15087/1/2002%20Soler%20Evoluci%C3%B3n%20la%20base%20de%20la%20Biolog%C3%ADa%20.pdf
Olkovaia, E. (2005) The unity of the world and symmetry. University of San Buenaventura, Colombia. Available at: https://www.redalyc.org/pdf/3435/343529891007.pdf
Solís-Marín, F. (2011). Crinoids, stars, brittle stars, sea urchins and sea cucumbers (Echinodermata). Available at: http://siaversedema.org.mx/wp-content/uploads/2018/04/publicacion/Biodiversidad%20Estudio%20Estado%20Vol%20II.pdf#page=224
