Why do all animals exhibit horizontal symmetry?

They don't. First off “horizontal” is not really a meaningful term in biology because position is determined relative to the animal, not on its environment. read more

They don’t. First off “horizontal” is not really a meaningful term in biology because position is determined relative to the animal, not on its environment. read more

Almost all animals exhibit some form of symmetry; the main exceptions are sponges, which are asymmetric in shape. The most common body plan is the bilateral symmetry seen in groups as diverse as worms, insects and vertebrates, while many simpler animals are radially symmetric, and a few (the echinoderms) are pentaradially symmetric. read more