Body symmetry is the description of the arrangement of a body and its parts and systems in a balance. The term symmetry implies that the body can be split in at least one way into equal, virtually identical halves. There are essentially four types of body symmetry known in the Animal Kingdom: asymmetry, bilateral symmetry, radial symmetry, and spherical symmetry.
Asymmetry, as the name implies, is a body that lacks any visible symmetry. (The prefix "a-" means "without.") Creatures with this type of symmetry are typically amorphous, with no discernible "left" or "right" side. There is no definite plane through which one can separate the body into equal parts.
The sponge (Phylum Porifera) is really the only animal that exhibits asymmetry. Unlike its famous cartoon counterpart (which exhibits another symmetry), a sponge resembles a loose, porous arrangement of tissue that seems to grow in random patterns. In most species, sponges eat, breathe, and excrete waste through the same pores and are fixed at the bottom. However, one cannot split a sponge into two mirror-images in any direction.
Another organism that exhibits asymmetry is everyone's favorite microorganism, the amoeba.
"Bi-" is a prefix meaning "two," so a bilaterally symmetrical animal is one whose body is capable of being split in two identical halves. Creatures with this type of body arrangement have clearly identifiable left and right, top and bottom, front and back sides. These also tend to have a progressive feature called "cephalization" where sensory organs are located near the "head" ("cepha-" means "head"), which aids in detecting danger all from one location. Bilateral symmetry brings with it a whole host of directional references, including anterior and posterior (head and bottom), and ventral and dorsal (front and back).
Most animals are bilaterally symmetrical. Humans exemplify this most obvious symmetry. If the human body was split right down the middle, from the top of the head, down between the eyes and downward, the end result would be two mirror-images. The typical human body has left and right eyes, hands, kidneys, and knees. The same goes for horses, alligators, eagles, frogs, and sharks (as well as the aforementioned cartoon sponge).
However, with bilateral symmetry, the body shapes are not capable of being separated laterally, say across the stomach. The top and bottom halves are not even close to being similar; hence, the "bi-" or "two" specification.
Just like the radius of a wheel or the "r" in A = Pi * r (squared), radial symmetry implies a bodily organization around a centralized point. The body parts seem to radiate from a centralized location, like from the middle of a circle or pizza pie. A cut through this central point of the body will result in identical halves.
Sea stars (more commonly but incorrectly known as starfish) exemplify radial symmetry. Another characteristic of animals in this category are a top and bottom half, the latter typically being the mouth and attachment point. Refer again to the sea star, whose means of living involves clinging to the ocean floor catching bottom-dwelling creatures with its suction arms and eating with its mouth. The top part is armored to protect it from over-dwelling predators.
Other radially symmetrical organisms include flowers and viruses.
Spherical symmetry obviously involves organization around a spherical body plan. This means a slice across any plane that passes through the center of the body will create two halves that are mirror-images of each other. These creatures are shaped like a ball or a globe.
There are no known animals with spherical symmetry, though the blastula or blastophere stage of early embryonic development is definitely spherically symmetric.
Among the many means of dividing the vast number of organisms on Earth into categories, body symmetry is another. It is useful in studying the advantages and adaptations of certain organisms to their particular environments. Indeed, much research has shown that a creature's body shape is at least correlated to and at most dictated by the environment in which it lives.