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| There are about 375 species of shark which can be placed in eight groups, or orders, according to the presence or absence of certain external or internal characteristics, such as anal fins, fin spines, shape of the valve in the gut, and so on. When classifying any group of animals, scientists usually try to work out which are more closely related to each other and put those in a group together. But it is not always possible to sort out all the relationships, so some may be grouped together just for convenience. Classification may change when new sharks are discovered or when further studies reveal new relationships. |