Biomineralized tissues and organs usually have a mechanical function. This is certainly true for the skeletal elements of vertebrates, which serve as lever anchors for muscles to produce motion in the limbs or as protective casings of internal organs such as the skull. Similarly, many (but not all) teeth are mineralized to provide the hardness and stiffness needed to tear and crush food substances. Biomineralized tissues also exist in many other forms in nature, such as the calcified cartilage of sharks, mineralized shells of snails and bivalves, the exoskeleton of crustaceans or spicules, and spines such as those of sea urchins.