How are sharks and rays different from fish?
Sharks have a lot of structural similarities to other fishes. However, there are some significant differences as well. The biggest difference is that while fishes have bony skeletons, sharks have skeletons made mostly of cartilage. Shark fins are usually thicker and stiffer and lack the spines of bony fish fins. The gills of sharks are visible externally while those of most bony fish are covered by a bony plate, called an operculum or gill cover.
