Animals are divided into two groups; vertebrates and invertebrates. Animal classification

Animals are divided into two groups; vertebrates and invertebrates. Animal classification is a matter of sorting out similarities and differences. This sorting process enables scientists to describe characteristics through the various levels of animal groups and subgroups. Among the most basic criteria by which animals are sorted is whether or not they possess a backbone. This single trait places an animal into one of two groups: the vertebrates (with backbone) or the invertebrates (without backbone). This represents a fundamental division among all animals.
What internal structure identifies an organism as an invertebrate?