Sharks are possibly the most feared yet most fascinating sea creatures. Of about 350 known species of shark, only a handful are a threat to humans, we are more of a threat to them, hunting and fishing them to the verge of extinction.
Sharks comprise only about 1 percent of all living fishes, and share all the major features. Like all fishes, sharks use gills to extract the oxygen from the surrounding water in which they live. Unlike most bony fish though, sharks don't have a swim bladder or well developed ribs, so they are slightly negatively buoyant. Therefore they must continually swim or they will sink slowly.
Swimming is achieved by a side-to-side movement of the tail, which creates forward propulsion. Sharks also share with other fishes nearly all the same general features of their internal anatomy, including circulatory, digestive, reproductive and nervous systems.
There are other features that separate sharks from fishes. Externally, all of the shark's gill slits are visible, where the gills of most other fish are covered and protected by a bony plate known as a gill cover. The sharks skin has a layer of tiny, but tough, dermal denticles, as opposed to the much larger flattened scales in most other fish.
Below is a diagram showing the external characteristics of a typical shark.
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