As you may have guessed from the name, members of the Chondrichthyes have a cartilaginous skeleton. The teeth, and in some cases the vertebrae, are hardened with calcium salts, but this calcified cartilage is not true bone. Cartilaginous fish also have thick, fleshy fins, unlike bony fish. The tail of cartilaginous fish is also distinctive: the tail is divided into two lobes, and the vertebrae extend into the upper lobe, which is elongated past the lower half. Such a tail is called heterocercal. In contrast, most ray-finned fish (except for primitive forms such as sturgeons and gars) have a symmetrical or nearly symmetrical tail fin -- a homocercal fin. Look carefully at the pictures below. Which one belongs to a shark? Click on the picture to find out.
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