In biology, symmetry is the repetition of body components of the body of a living organism in an ordered manner. Specifically, symmetry refers to the correspondence between body parts in terms of shape, size, and relative positions, in opposite directions of a division line or around an axis or central point. Except for circular symmetry, the form of the body does not have any relation to internal anatomy since animals with very distinct anatomy may share the same kind of symmetry.
Animal Characterization based on Body Symmetry
At the most basic level of classification, real animals are generally classified into three categories based on the symmetry type of their body plans that is radially symmetrical, bilaterally asymmetrical, and bilaterally. Only a handful of animal species have radial symmetry. It is the only characteristic of the species Porifera (sponges). Each type of symmetry is suitable for the specific requirements of an animal’s life.
Radial symmetry refers to an arrangement that arranges body components along a central axis as rays of sunlight or the pieces of the pie. Radially symmetrical animals have both top and bottom surfaces but have no right and left sides or front and back. The two parts of a radially symmetrical animal can be identified as the side that has a mouth (“oral side”) and the other side that does not have a mouth (“aboral side”). This type of symmetry is evident in the body plan of animals within the species Ctenophora (comb jellies) and Cnidaria (corals sea anemones, corals, and other jellies). Radial symmetry allows these sea creatures, which might be passive or capable of slow motion and floating, to see the world similarly at every angle.
Symmetry in flowers
The notion of symmetry can also be used in botany. A flower is considered symmetrical when each whorl is composed of the same number of components or when the details of any whorl are greater than those preceding it. An asymmetrical flower could have five sepals, five stamens, five petals, and five carpels; the total number of these parts can be more than five.
The amount of parts within the pistillate (female) whorl is usually not the same as in other whorls; however, in such instances, the flower can still be described symmetrically as long as all the different whorls have normal. Flowers in which the parts are split into twos are dimerous. In threes, fours, or fives, it is trimerous or tetramerous and pentamerous. Trimerous symmetry is the norm for monocotyledons. Pentamerous is the most frequent in dicotyledons. Dimerous and tetramerous blooms are also found in the latter category.
When the various members of a whorl are identical, the flower becomes regular and is known as actinomorphic or similar to the petunia, buttercup, and wild rose. Different sizes or shapes of the components of a whorl can make the flower unnatural (as in the case of canna and Asiatic dayflower). If a flower is divided by an individual planar plane into two equally sized parts, it’s Zygomorphic or bilaterally symmetrical such as the snapdragon and orchid or sweetpea.
All members in the family of phylum Porifera (sponges) cannot show Body plan symmetry. A few species of fish, like flounder, lack symmetry in adulthood. However, the fish that are larval have bilaterally symmetrical.