Invertebrates are all species of animals without a backbone or spine, which includes arthropods (insects, crustaceans, etc.), mollusks (bivalves, snails, squid, and octopus, etc.), annelids (worms, etc.), and cnidarians (corals, anemones, jellyfish, etc.). The earliest fossils appear to be invertebrates which have been dated back to 665 million years ago. Invertebrates are the largest class of animals, comprising 97% of the animal kingdom with 1.25 million species discovered, but millions still to be discovered. A key to the large quantity of invertebrates is their ability to reproduce quickly and abundantly, with some species’ members producing both egg and sperm, and some species laying eggs that do not require fertilization. Invertebrates are comprised of a wide range of species, all the way from microscopic mites to the giant squid.