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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.

Invertebrates are ectotherms (cold-blooded): they warm their bodies by absorbing heat from their surroundings. Most invertebrates live in water or spend at least some part of their life in water. The external layers of aquatic invertebrates are generally thin and permeable to water. This structure allows the ready exchange of gases needed to keep the animal alive. Some aquatic vertebrates do have...