Animals that do not have a backbone are classified as invertebrates. This group includes a diverse range of species such as insects, arachnids, mollusks, and crustaceans. Unlike vertebrates, which have a spine made of vertebrae, invertebrates often possess exoskeletons or hydrostatic skeletons. They make up about 95% of all animal species on Earth and play vital roles in ecosystems, such as pollination, decomposition, and serving as a food source for larger animals. Common examples of invertebrates include jellyfish, snails, octopuses, and spiders, illustrating the extensive variety and adaptability of these creatures in various habitats.