How many species have internal skeletons?

Internal skeletons are characteristic of vertebrates, which include animals such as mammals, birds, reptiles, amphibians, and fish. There are about 68,000 known species of vertebrates. Additionally, some invertebrates, like certain species of echinoderms (e.g., sea stars and sea urchins), also possess internal skeletons made of calcareous materials. Overall, the total number of species with internal skeletons is in the range of tens of thousands, primarily dominated by vertebrate species.