Animals without a backbone are called invertebrates. These organisms lack a spinal column and cranium base in their body structure. There are over 1 million known species of invertebrates.
Invertebrates can be grouped into eight taxonomic classes. These classification groups, also known as phyla, include cnidarians, porifera, platyhelminths, nematode, echinoderms, molluscs, arthropods and annelids. Within each of these classifications are sub-groups that are based on characteristics of the organisms. For instance, the animals categorized as arthropods can further be divided into sub-groups such as insects, crustaceans and arachnids. Being heterotrophs, invertebrates consume both plants and other animals for their sustenance.