Shark eggs, unlike those of most fish, are fertilized inside the female's body. The male shark has two organs called claspers, which release sperm into the female, where it fertilizes the eggs. Among most species of sharks, the eggs hatch inside the female, and the pups are born alive. At least 40 species lay their eggs outside their bodies. Tail and fins.
Sharks can travel with great bursts of speed when excited.
The upper part of the tail usually is longer than the lower part. Most fish have a swim bladder, a gas-filled organ that helps them remain at a certain depth without sinking. Sharks lack this organ. Instead, they have a large liver filled with oil. This oil is lighter than water, and it helps the shark keep from sinking.