Sponges are the only type of animals without radial or bilateral symmetry.
In the animal kingdom, there are three primary types of symmetry. Asymmetry, no symmetry at all, exists only in the simplest animals, which are considered sponges. Radial symmetry, symmetry along multiple planes, exists in cnidarians. Bilateral symmetry, symmetry along one sole plane, exists in creatures as simple as flatworms and in those as complex as humans.
Radial Symmetry
Radial symmetry is rotational symmetry that focuses around a point known as the center. This means that everything at the same distance from the center is the same. Organisms with radial symmetry can be cut in half in multiple places and the two resulting bodies would be exactly the same. Radial symmetry can be divided into two distinct classes. Cyclic symmetry has only rotational symmetry, while dihedral symmetry has rotational symmetry and reflection symmetry, a phenomenon in which two sides exactly mirror one another.
Examples of Radial Symmetry
Animals such as starfish can be cut evenly along multiple planes.
Radial symmetry is most often seen in animals in the cnidarian phylum, which includes more than 9,000 species of animals that live in aquatic environments. Perhaps the best known example of radial symmetry is a starfish. Starfish have dihedral symmetry with legs spanning out and forming an even pentagon-like shape. Another example of radial symmetry exists in jellyfish. They can be cut evenly along six different planes. Plants such as the hibiscus occasionally have radial symmetry as well. At times, radial symmetry in plants is harder to detect because parts of the plant may cover up visible traces of radial symmetry.
Bilateral Symmetry
Bilateral symmetry did not evolve on Earth until well after the birth of radial symmetry animals. Animals with bilateral symmetry can only be evenly cut down one plane, as opposed to the multiple planes in radial symmetry. Usually, the cut exists along the longitudinal axis of the animal. The longitudinal axis typically extends from an animal's mouth to its tail-end.
Examples of Bilateral Symmetry
Human faces display bilateral symmetry.
Bilateral symmetry exists in all animals that are not classified as sponges, which do not have any type of symmetry, or cnidarians. Examples of organisms that do have bilateral symmetry include animals classified as flatworms, mollusks, annelids, roundworms, arthropods, echinoderms and chordates. A good example of an animal with bilateral symmetry is an opossum. If it were cut down its back, the resulting bodies would be exactly the same. Larger species such as elephants and humans also have bilateral symmetry along their backs.
Tags: multiple planes, radial symmetry, rotational symmetry, symmetry along, symmetry exists, would exactly same
