Sharks have a spinal column and are thus vertebrates. However, their skeleton is not made of bones, but of cartilage, and together with their nearest relatives the rays and the chimeras they form the class of cartilaginous fish. Fish such as eels, trout, cod or tuna have a bony skeleton and thus belong to the class of bony fish. Sharks differentiate themselves both physiologically and anatomically from fish, for example through their ampullae of Lorenzini and their valvular intestine, although these differences are not so obvious from the exterior.