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Fish (plural: fish or fishes) are a group of animals with bones which live in water and respire (get oxygen) from their gills. Fish are vertebrates. They all have a form of backbone.
There are more fish than four-limbed animals: there are over 33,000 described species of fish. Fish are usually covered with scales. They have two sets of paired fins and several unpaired fins. Most fish are cold-blooded (poikilotherm).
There are many different kinds of fish. They live in fresh water in lakes and rivers, and in salt water in the oceans. Some fish are less than one centimetre long. The largest fish is the whale shark, which can be almost 15 metres long and weigh 15 tons. Almost all fish live in the water. A group of fish called the lungfish have developed lungs because they live in rivers and pools which dry up in certain parts of the year. They burrow into mud and aestivate (sleep all summer) until the water returns.
Some fish are more closely related to land animals than they are to other fish. For example, lobe-finned fish were the first animals with bones to come live on land, and all land animals are their descendants. Lobe-finned fish are more closely related to humans than to ray-finned fish.
“Fish” is not a formal grouping in biology. Amphibians, reptiles, birds and mammals all descended from lobe-finned fish (and not from fish as a whole). But the use of the term “fish” is so convenient that we go on using it.
Fish are the oldest vertebrate group. The term includes a huge range of types, from the Middle Ordovician age, about 490 million years ago, to the present day.
Certain animals that have the word fish in their name are not really fish: crayfish are Crustacea, and jellyfish are Cnidaria. Some animals look like fish, but are not. Whales and dolphins are mammals, for example.
Most kinds of fish have bones. Some kinds of fish, such as sharks and rays, do not have real bones. Their skeletons are made of cartilage, and so they are known as cartilaginous fish.
All fish are covered with overlapping scales, and each major group of fish has its own special type of scale. Teleosts (‘modern’ fish) have what are called leptoid scales. These grow in concentric circles and overlap in a head to tail direction like roof tiles. Sharks have scales made of denticles, like small versions of their teeth. These also overlap in a head to tail direction, producing a tough outer layer. Shark skin is available for purchase as shagreen, a leather which is smooth in one direction, and rough in the other direction. It may be polished for use, but is always rough in texture and resistant to slipping.
The scales are usually covered with a layer of slime which improves passage through the water, and makes the fish more slippery to a predator.


What is the meaning of poikilotherm? (line 6)