Have you ever noticed that most animals seem to be symmetrical? You can draw a line down the middle of their bodies, and each half of their body looks the same. Look in a mirror—you're symmetrical too.
Two by Two
Many living creatures exhibit what scientists call bilateral symmetry. This means that if you divide their bodies in half, the two sides appear to be mirror images of each other. Scientists believe that bilateral symmetry first developed about 550 million years ago. Today, many kinds of animals, including arthropods, flatworms, mollusks, and all vertebrates demonstrate bilateral symmetry. Scientists aren't quite sure why this form of symmetry is so common in the animal kingdom. They think it might have something to do with the evolution of the central nervous system. Bilateral symmetry may also make movement easier.
Not all animals have bilateral symmetry, however. Some sea creatures, such as jellyfish, starfish, and sea anemones, have rotational symmetry instead. And a few animals, like sponges, aren't symmetrical at all.
See for yourself: http://evolution.berkeley.edu/evolibrary/article/arthropods_04
How many animals can you name that demonstrate bilateral symmetry?