Fish are ancient water-dwelling creatures found all over the globe. They are the world's oldest vertebrates, with the first fish appearing some 500 million years ago. Today more than 24,000 species populate the waters of the world. Fish are vertebrates and breathe using gills, which draw oxygen from the water and into the bloodstream. Most have bony skeletons, though sharks and rays have skeletons made of cartilage. Nearly all reproduce by laying eggs. (source:animals.nationalgeographic.com)