Invertebrates are animals without backbones. This simple definition hides the tremendous diversity found within this group which includes protozoa (single-celled animals), corals, sponges, sea urchins, starfish, sand dollars, worms, snails, clams, spiders, crabs, and insects. In fact, more than 98 percent of the nearly two million described species are invertebrates. They range in size from less than one millimeter to several meters long. Invertebrates display a fascinating diversity of body forms, means of locomotion, and feeding habits.