The difference between invertebrates and vertebrates is that a vertebrate is an animal with a backbone or spinal column and an invertebrate is an animal without these. There are around 50,000 known species of vertebrates including all fish, birds, reptiles, amphibians and mammals. Most vertebrates are much more advanced and usually larger than invertebrates. They are also often much smarter than invertebrates and have complex nervous systems. Vertebrates have muscles and skeletons as well and this allows them to move in complicated ways. The largest animals to ever exist on the planet were vertebrates and the most simple to ever exist (the sponge) is an invertebrate.
There are many more species of invertebrates than there are vertebrates as they make up 97% of all animal species alive today. There are over 1.3 million invertebrates in the world including all protozoa, coelenterates, flatworms, annelid worms, echinoderms and arthropods (arachnids, crustaceans, insects and myriapods). Invertebrates evolved from single-celled micro-organisms around 600 million years ago and they were the first animals to evolve. As they don’t possess backbones, many invertebrates have an alternative way to have structural stability. This may be by having an exterior shell or exoskeleton. The majority of invertebrates change form as they grow and lots form large colonies.