Invertebrates are animals without backbones. Most of the world’s animals are invertebrates but most of the big, obvious animals are vertebrates. Why? Because the vertebrate bone structure allows animals to attain much larger sizes than is possible in the invertebrates. So although we are not as common or diverse, we are definitely the dominant group size-wise.
The most numerous invertebrates are single celled animals. After that come a variety of invertebrate phyla: sponges, jellyfish, worms and molluscs are all invertebrates. The biggest, most diverse invertebrate phylum is the Arthropoda. These animals have gone for an exoskeleton which has given them some advantages over the other invertebrate phyla. The exoskeleton is a hard outer covering that protects the animal from predators and to which the muscles are attached on the inside and from which the jointed legs are attached on the outside. Spiders and mites, insects and crustaceans are all arthropods and together they comprise the most diverse and numerous phylum on earth. There are from one to thirty million species of insects alone!
However the arthropod body plan does not allow for large size. The largest arthropods were the giant dragonflies of the Devonian period, which had three meter wing spans. But there is a limit to the size they can attain because of mechanics. Attaching muscles to the inside of the skeleton is much more limiting than having the muscles on the outside of the skeleton, which is the solution that vertebrates have adapted. The other major limitation of the arthropod body plan is that the exoskeleton must be shed in order for the animal to grow. This is a problem vertebrates do not have to face. Our bones and bodies can keep growing without the difficulties and dangers associated with moulting.
Vertebrates developed an internal skeleton which is based first on the spinal chord. Some members of the Phylum Chordata stopped there. Amphioxus is a worm with a spinal chord but no bones or skeleton. Sharks took the design a step further and encased the spinal chord in protective cartilage but fish went the whole way with the creation of hard bones. These bones not only encased the brain and spinal chord but went on to evolve into limbs, which eventually led to land vertebrates: amphibians, reptiles, mammals and birds. Different as they are, they all share a common underlying structure of bones with the vertebral column being central.
The vertebrates are unique in that they have an endoskeleton that supports them against gravity and allows them to grow large, moveable bodies that have adapted to numerous habitats. Insects can fly but bats and birds dominate the air. Insects and crustaceans can crawl but fish and mammals dominate the seas and the land. The invertebrates form the lower parts of food chains but the vertebrates dominate at the top. Eagles, tigers, humans and whales have all reached the tops of their food chains because of their vertebrate structure.