The number of fish species runs to more than 20, 000 in the entire world! They can be classified into three broad categories: bony, cartilaginous and jawless. These species are spread across various biospheres. Around 10,000 species are found in freshwater; whereas, around 20,000 live in saltwater. There are also fish species that can live both in fresh and salt Water.

               The most commonly found fishes fall in the category of bony fishes. Tiny bones make up the skeleton of these fishes. They also have scales all over their body. They breathe through gills that appear at both sides of their head.

                On the other hand, cartilaginous fish have skeletons made out of cartilage. Cartilage is a kind of flexible tissue, and it is less brittle than bone. Sharks and rays have cartilages instead of brittle bones. As you may have observed, sharks have several gill slits on either side of their heads. They do not have scales like fishes. However, they have little interlocking teeth that make up its tough skin.

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