The endoskeletons and exoskeletons of various animals are also formed from :

The carbon compound which forms the endoskeleton and exoskeleton of animals is calcium carbonate. Seashells are the exoskeletons of mollusks such as snails, clams, oysters and many others. Such shells have three distinct layers and are composed mostly of calcium carbonate with only a small quantity of protein--no more than 2 percent.