Invertebrates are those animal species which do not belong to the Sub-Phylum Chordata, creatures with backbones. Put crudely, invertebrates lack a dorsal spinal chord with the nerves running through it and the bony or cartaliginous skeleton possessed by fish, amphibians, reptiles, birds and mammals. Despite this apparent disadvantage, a staggering 95% of living creatures on the earth are invertebrates, from single-celled creatures such as amoebas (admittedly a rather borderline case for the animal kingdom) through sponges, anemones, jellyfish, various parasitic worms, annelid worms, arrow worms and other little-seen creatures up to the insects, the arachnids, the crustaceans and the molluscs, two of whose representatives, the octopus and the squid, are actually fairly intelligent and have developed certain features (such as the form of the eye) normally found only among vertebrates.
Apart from the sheer number of invertebrate species, their diversity is enormous. Molluscs, for example, form a zoological phylum of their own and range from the sedentary brachiopods and mussels up to the giant squid. They number about 300,000 different species but this number pales into insignificance besides the insects, of whom at least 800,000 species are recognised, a total that in truth is probably at least one million. Most if not all of these families are also very old if not ancient, some with roots going back to the Cambrian era of about 500 million years ago. The horseshoe crab Xanthippus is a living relative of the once-abundant trilobite and is at least 300 million years old. Arachnids, in the form of spiders and scorpions, conquered the land some millions of years before the first lungfish appeared. Insects are actually more recent to the history of life on earth but are now so hugely significant that much of life depends directly or indirectly on their activity, both as items in the food chain and as pollinators of plants.
The term 'spineless' has become one of derision in human society when applied to a human being, but in fact most of the invertebrates have developed their own form of body structure which grants them stability and structure. Worms tend to have a protective cuticle around them, while arachnids, crustaceans and insects rely on an exoskeleton, in essence having a hard shell or outer layer which is periodically shed as they grow. Marine invertebrates tend to rely less upon such structures, the water supporting them, but even here lobsters and crabs have exoskeletons while cephalopods usually have a cartaliginous pen, a sheath-like structure inside their body, that gives them internal support.