Animals with endoskeletons include many plant species, birds and humans. The global animal kingdom includes species with external skeletons, called exoskeletons, and organisms with internal skeletons, called endoskeletons. Creatures with endoskeletons contain skeletons formed from sturdy bones; they have a complete skeletal structure on which muscles, tendons and skin rests, while animals with exoskeletons contain outer shells with tissues, tendons and vital organs residing in soft compartments beneath those shells.
Many mammals and fish also have endoskeletons. Internal skeletons support life in essential ways for many organisms. In addition to providing structure and stability, these skeletons grow along with the living creatures. Unlike organisms with external skeletons, animals with endoskeletons never shed their skeletons when passing through various life stages. Their skeletons remain intact inside their bodies, creating critical frames around vital organs and keeping those organs in place.
Skeletons also serve as attachments for ligaments and tendons, which affix to the outside layers. Internal skeletons consist of many joints and hinges, giving organisms freedom of movement.
Endoskeletons take several different shapes, giving unique advantages to living creatures. Some derive from thick but flexible cartilage, while others form from hard bones. Some skeletons, like those in humans and sharks, contain a combination of bones and cartilage, making creatures adept for life in particular environments.