This is the scientific name for the animals. Oddly, the animals are not well defined. The evidence that the sponges and the animals with epithelia are related, allow us to define the animals to include sponges, cnidaria andall epitheliate animals. The animals are distinguished as organisms which may be multicellular, use extracellular collagen as a skeletal material, have a sexual developmental cycle that involves motile sperm, relatively immotile eggs, and development that involves the formation of a blastula. With our current understanding, this life form has diversified much more than any other group. Animals are often divided into the vertebrates (including fish, amphibia, reptiles and birds, and mammals), and the invertebrates.