^w^ A shark is a kind of fish, but it has evolved very differently than the 25,000 species of bony fishes.
^w^ The cartilaginous skeleton is the major distinction between sharks and bony fishes, but their respiratory system, liver, skin, teeth, jaw structure, and ability to adjust to salt water also make sharks unique.
^w^ Sharks have developed an internal mechanism that adjusts the salt content of their blood so that it is nearly equal to sea water. This means that they do not have to drink water to
fight dehydration as do other fishes and people.
^w^ Instead of excreting all the toxic urea like other vertebrates, sharks keep the urea at high concentrations in their blood so their internal saltiness matches the salt level in the water.
^w^ Sharks are very healthy. Parasites are one of the few health problems they encounter.
^w^ Most sharks are cold blooded. But some, like Great Whites and Makos, keep the temperature in their vital organs, especially their brains, several degrees warmer than the surrounding water.
