Invertebrates are animals without a backbone or bony skeleton. They range in size from microscopic mites and almost invisible flies to giant squid with soccer-ball-size eyes.
This is by far the largest group in the animal kingdom: 97 percent of all animals are invertebrates. So far, 1.25 million species have been described, most of which are insects, and there are millions more to be discovered. The total number of invertebrate species could be 5, 10, or even 30 million, com- pared to just 60,000 vertebrates.
One reason for the success of invertebrates is how quickly they reproduce. Sponges and corals, for example, produce both eggs and sperm. Social insects such as ants and bees lay eggs that can develop without fertilization—they become the workers.
Insects in particular are successful because they are so adaptable. They are opportunistic eaters, feeding on plants, animals, and decaying organic material. They are able to survive in extreme environments, including very hot, dry habitats. And many can fly—either to escape predators or to find new sources of food, water, and shelter.
Like vertebrates, invertebrates are classified based upon their body structure, life cycle, and evolutionary history.
—From the National Geographic