The Definition of a Symmetric Body Plan

Asymmetry

• Symmetry is useful in organisms; it helps to streamline the organization of their various internal systems, and it adds a sense of direction. Once an organism has an axis, it also tends to move in one identifiable direction, which over time, leads to the evolution of a head containing most of the sensory organs. As a result, very few animals are truly asymmetric; one notable exception is the sponge group of organisms, some of whom grow with no clearly definable body plan whatsoever. Notably, they also lack true nervous or digestive systems.

Bilateral Symmetry

• Among animals, the most common form of symmetry is bilateral: if you divide an animal down its centerline, you wind up with two roughly symmetric halves. Symmetry, in biological terms, is generally imperfect; for example, human beings are bilaterally symmetric, with two lungs, two arms, two legs, two kidneys, two eyes and a brain with two halves. But single organs like the heart and stomach do not cleanly divide along the midsection.

Radial Symmetry

• Some animals, generally oceangoing ones, exhibit symmetry that divides them into more than two parts around a central axis. For example, if you've ever looked at jellyfish washed up on the beach, you might've noticed that they appear to be divided into quarters or even fifths. Starfish and their relatives, like sea urchins, are frequently pentaradial: having five equal symmetric parts. In starfish, this symmetry is so complete that if you cut one arm off, it can regenerate an entirely new starfish -- no doubt providing quite a surprise for oyster fishermen who tried to kill off their starfish competitors by cutting them up.

Spherical Symmetry

• In much rarer cases, some creatures exhibit spherical symmetry; they can be cut along many axes, not simply different cuts along the same centerline. The radiolarian protozoa, for example, have mineralized skeletons that form regular polyhedral structures. Each face of the protozoan is symmetric to all the others. Heliozoa are also roughly spherical; heliozoans are outwardly similar to radiolarians, but are less complex overall and lack their skeletons.