Within the vertebrates are the groups Agnatha and Gnathostomata. This class is not extremely diverse, having only 750-900 species. The sharks alone comprise over 350 of those species. Their habitat is exclusively aqueous, though chondrichtyes can live in any waters from cold, polar oceans to warm, tropical waters. Chondrichthyes are heterotrophic , and they can be carnivorous, omnivorous, or herbivorous.
Most sharks are carnivorous, though, and can swallow their prey whole.
Their diets consist primarily of fish, squid, clams, and other small marine mammals. Skates, rays, and chimaeras feast mostly upon crustaceans and mollusks. Some chondrichtyes are scavengers, feeding on injured fish and garbage. Ironically, while the smaller sharks stick to the previously described diet, the largest kinds, such as the whale shark and the basking shark, eat plankton and tiny plants. Food is stored in a J-shaped stomach, and though the chondrichtyes' intestines are short, they acheive increased surface area through use of a spiral valve.