A body part is called a jaw based on what it does, not what it is made of or how it grows. In different animals, it can be made of different kinds of cells and tissue. In the embryos of different animals, it may come from different original places. Even within arthropods, the group that contains insects and lobsters and trilobites and spiders, the jaws of different animals may grow from different original tissues, even though they end up looking similar in adult animals. A pair of appendages that become jaws in one species may become antennae in a different species or legs in another. Moray eels and some other fish have two sets of jaws. The outer jaws in their faces grow from the same tissues as the jaws of other fish, and the inner jaws, called pharyngeal jaws, can come from the same tissue that becomes gills or muscles in other fish.