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, the face of a human being has a plane of symmetry down its centre, or a pine cone displays 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.
A selection of animals showing a range of possible body symmetries, including asymmetry, radial, and bilateral body plans
Illustration depicting the difference between bilateral (Drosophila), radial (actinomorphic flowers) and spherical (coccus bacteria) symmetry
Lilium bulbiferum displays hexamerism with repeated parts arranged around the axis of the flower.
Apple cut horizontally showing that pentamerism also occurs in fruit
Placozoa is a phylum of marine and free-living (non-parasitic) animals. They are simple blob-like animals without any body part or organ, and are merely aggregates of cells. Moving in water by ciliary motion, eating food by engulfment, reproducing by fission or budding, placozoans are described as "the simplest animals on Earth." Structural and molecular analyses have supported them as among the most basal animals, thus, constituting a primitive metazoan phylum.
Crawling motility and food uptake by Trichoplax adhaerens
