A shark is a fish that lacks bones and instead its skeleton is made of cartilage. A fully grown shark is about 18 cm long however, most are about 1.5 to 2.1 metres long. It has a streamlined body enabling it to puddle through water.
Ancient relatives of sharks first appeared in the world's oceans about 400 million years ago. Today, sharks are classified among the elasmobranchs, which also includes the rays and skates. There are over 350 known species...
There are so many different types of sharks in the world that it's hard to say what they all look like. Most sharks move very quickly, unlike fish. They all have sets of replaceable teeth. They used to be known as sea dogs before they were called sharks.