Incredible Diversity: The one thing that all invertebrates have in common is their lack of a backbone; otherwise, they're as different from one another as they are from vertebrates. All vertebrates fall under one phylum, chordata; invertebrates, on the other hand, include over 30 different phyla, collectively accounting for 98 to 99 percent of all known animal species. The major invertebrate phyla are porifera (sponges), platyhelminthes (flatworms), nematoda (round worms), annelida (earthworms, marine worms and leeches), cnidaria (jellyfish, coral and sea anemones), mollusca (octopus, nautilus, squid, slugs, snails and bivalves), arthropoda (insects, spiders, scorpions and crustaceans) and echinodermata (starfish, sea urchins and their relatives).

Abundant Arthropods: More than three-quarters of the world's known animal species are arthropods. They include around one million species, around 90 percent of which are insects. The other major arthropod classes are arachnids, which include spiders and scorpions, and crustaceans, which include crabs, lobsters and barnacles. Arthropods are everywhere: on land, in the air and beneath the water's surface. They all have segmented, jointed bodies and are covered by a hard exoskeleton made of chitin.

Flat, Round and Segmented Worms: Worms are long, soft-bodied animals with no legs. They live in all types of environments, even as parasites inside plants and other animals. Many of the 25,000 species of flatworm — flat, ribbon- or leaf-shaped animals — are parasitic, including the familiar tapeworm. There are over 80,000 species of roundworm, 15,000 of which are parasitic. Some of the free-living species live in extreme environments like Antarctica and oceanic trenches. There are about 15,000 species of segmented worm, including the familiar earthworms and leeches. These worms — whose bodies are formed by repeated, ring-like structures — also include polychaete or bristle worms, which live mainly in the ocean, and tubeworms, which live along deep-sea vents and seeps.