No. They have gills.
Gills consist of dense networks of thin-walled blood vessels over which water is pumped. This allows carbon dioxide in the blood to move into the water, and oxygen from the water to move into the blood.
The gills are arranged into a "counter current" system so the water flows through in the opposite direction to the blood. This means that oxygen will always be picked up via the blood, and carbon dioxide will always be picked up by the water.
The oxygen gets into the water in the first place in several ways. The most important sources are marine plants, which photosynthesise and produce oxygen as a by-product, and also oxygen from the air dissolving in the water.
The carbon dioxide entering the water, on the other hand, makes perfect plant food - the marine plants and plankton absorb it and, again using photosynthesis, turn it into sugars and oxygen
-Courtesy of thenakedscientists.com
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