Science, Tech, Math › Animals & Nature Facts About Invertebrates Share Flipboard Email Print Science, Tech, Math Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Marine Life Forestry Dinosaurs Evolution View More By Bob Strauss Science Writer B.S., Cornell University Bob Strauss is a science writer and the author of several books, including "The Big Book of What, How and Why" and "A Field Guide to the Dinosaurs of North America." our editorial process Bob Strauss Updated August 19, 2019 Ask a friend to name an animal and she'll probably come up with a horse, an elephant, or some other kind of vertebrate. The fact is, though, that the vast majority of animals on earth—insects, crustaceans, sponges, etc.—lack backbones, and are thus classified as invertebrates. There Are Six Basic Invertebrate Groups iStockphoto The millions of invertebrate animals on our planet are assigned to six main groups: arthropods (insects, spiders and crustaceans); cnidarians (jellyfish, corals and sea anemones); echinoderms (starfish, sea cucumbers and sea urchins); mollusks (snails, slugs, squids and octopuses); segmented worms (earthworms and leeches); and sponges. Of course, the variation within each of these groups is so wide—scientists who study insects aren't much interested in horseshoe crabs—that professionals tend to focus on specific invertebrate families or species. Invertebrates Do Not Have Skeletons or Backbones Christopher Murray / EyeEm / Getty Images Whereas vertebrates are characterized by the vertebrae, or backbones, running down their backs, invertebrates completely lack this feature. But this isn't to imply that all vertebrates are soft and squishy, like worms and sponges: insects and crustaceans support their bodily structures with hard external structures, called exoskeletons, while sea anemones possess "hydrostatic" skeletons, sheets of muscle supported by an internal cavity filled with fluid. Bear in mind, however, not having a backbone doesn't necessarily mean not having a nervous system; mollusks, and arthropods, for example, are equipped with neurons. The First Invertebrates Evolved a Billion Years Ago up close with nature / Getty Images The earliest invertebrates were composed entirely of soft tissues: 600 million years ago, evolution had yet to hit on the idea of incorporating ocean minerals into exoskeletons. The extreme age of these organisms, combined with the fact that soft tissues were almost never preserved in the fossil record, leads to a frustrating conundrum: paleontologists know that the earliest preserved invertebrates, the ediacarans, must have had ancestors stretching back hundreds of millions of years, but there's no way to adduce any hard evidence. Still, many scientists believe that the first multicellular invertebrates appeared on earth as far back as a billion years ago. Invertebrates Account for 97 Percent of All Animal Species Chris Stein / Getty Images Species for species, if not pound for pound, invertebrates are the most numerous and widely varied animals on earth. Just to put things in perspective, there are about 5,000 mammal species and 10,000 bird species; among invertebrates, insects alone account for at least a million species (and possibly an order of magnitude more). Here are some more numbers, in case you're not convinced: there are about 100,000 species of mollusks, 75,000 species of arachnids, and 10,000 species each of sponges and cnidarians (which, by themselves, pretty much outclass all the earth's vertebrate animals). Most Invertebrates Undergo Metamorphosis www.victoriawlaka.com / Getty Images Once they hatch out of their eggs, the young of most vertebrate animals look just like the adults: all that follows is a more-or-less steady period of growth, That's not the case with most invertebrates, whose life cycles are punctuated by periods of metamorphosis, in which the full-grown organism winds up looking very different from the juvenile. The classic example of this phenomenon is the transformation of caterpillars into butterflies, via the intermediate stage of the chrysalis. (By the way, one group of vertebrates, the amphibians, do undergo metamorphosis; witness the transformation of tadpoles into frogs.) Some Invertebrate Species Form Large Colonies Inigo Cia / Getty Images Colonies are groups of animals of the same species that remain together throughout most of their life cycle; members divide up the work of feeding, reproducing, and sheltering from predators. Invertebrate colonies are most common in marine habitats, and the individuals are joined to the extent that the entire aggregation can seem like one giant organism. Marine invertebrate colonies include corals, hydrozoans, and sea squirts. On land, the members of invertebrate colonies are autonomous, but still joined together in complex social systems; the most familiar colony-forming insects are bees, ants, termites, and wasps. Sponges Are the Simplest Invertebrates Global_Pics / Getty Images Among the least evolved invertebrates on the planet, sponges technically qualify as animals (they're multicellular and produce sperm cells), but they lack differentiated tissues and organs, have asymmetrical bodies, and they're also sessile (rooted firmly to rocks or the seafloor) rather than motile (capable of movement). As for the most advanced invertebrates on the planet, you can make a good case for octopuses and squids, which possess large and complex eyes, a talent for camouflage, and widely diffused (but well-integrated) nervous systems. Virtually All Parasites Are Invertebrates NNehring / Getty Images In order to be an effective parasite—that is, an organism that exploits the life processes of another organism, either weakening or killing it in the process—you have to be small enough to climb into that other animal's body. That, in a nutshell, explains why the vast majority of parasites are invertebrates—lice, roundworms, and nematodes are sufficiently tiny to infest specific organs in their unfortunate hosts. (Some of the smallest parasites, like amoebas, aren't technically invertebrates, but belong to a family of single-celled animals called protozoans or protists.) Invertebrates Have Widely Varied Diets Michael Layefsky / Getty Images Just as there are herbivorous, carnivorous and omnivorous vertebrate animals, the same range of diets is enjoyed by invertebrates: spiders eat other insects, sponges filter small microorganisms from the water, and leaf-cutter ants import specific types of vegetation into their nests so they can cultivate their favorite fungus. Less appetizingly, invertebrates are also crucial for breaking down the carcasses of larger vertebrate animals after they die, which is why you'll often see the corpses of small birds or squirrels covered by thousands of ants and other icky bugs. Invertebrates Are Extremely Useful to Science Vaclav Hykes / EyeEm / Getty Images We would know much less about genetics than we do today if it weren't for two widely studied invertebrates: the common fruit fly (Drosophila melanogaster) and the tiny nematode Caenorhabditis elegans. With its well-differentiated organs, the fruit fly helps researchers decode the genes that produce (or inhibit) specific anatomical traits, while C. elegans is composed of so few cells (a little over 1,000) that the development of this organism can easily be tracked in detail. In addition, recent analysis of a species of sea anemone has helped to identify 1,500 essential genes shared by all animals, vertebrates and invertebrates alike.