鱼类终生在水中生活，体表有鳞，用鳃呼吸，用鳍游泳，心脏有一个心房，一个心室，一条血液循环路线。在水中产卵，体外受精。鱼类的体型多数为纺锤型或扁平型、棍棒型，这样能减少在水中游泳的阻力。背部颜色深，腹部颜色浅，有利于隐蔽自己。
鳍是鱼类维持身体平衡、控制运动方向、推动身体向前运动的器官。
在食道的背侧，还有鱼类所特有的一个器官——鳔。鳔内充满了气体，随着鳔的收缩或膨胀，引起身体比重的改变，使鱼能够浮沉或保持静止状态。
鱼类有特殊的皮肤感觉器官——侧线。侧线位于身体两侧的皮肤下面，一侧一条，通过鳞片以小孔与外界相通，侧线管内有感觉细胞，能感受水压和水流方向。
鱼类在脊椎动物中种数最多，现存种类约2万种。鱼类富有营养价值，肉味鲜美，除食用外，还可以提供各种工业原料。
What kind of animal can live in hot desert spring water or in the freezing water of the Antarctic？What animal can live in a shallow mud pond or the deepest part of the sea？Fish can.Antarctic icefish can live in water below 32°Fahrenheit (0°Celsius).Desert pupfish can live in water that is hotter than 100°Fahrenheit (40°Celsius).
Fish are animals that live and breathe in water.Some fish live in the fresh water of lakes and streams.Some fish live in the salt water of oceans.A few fish can live in both kinds of water.
All fishes are vertebrates (animals with backbones).Most fish have a covering of scales to protect their bodies.
A fish is an animal that lives in the water.Most fish swim by moving their tail fin left and right.A fish uses a special sense organ called the lateral line to feel nearby objects before it can see them
The walking catfish lives in shallow ponds in eastern India and Southeast Asia.When these ponds dry up，the catfish travels across land to deeper ponds.
In order to breathe underwater，most fish use gills.The gills remove oxygen from the water as the water passes over them.
Many fish swim together in groups，known as schools.
Most fish eat other fish or smaller animals.Some fish eat plants.Here，a blue cod eats a smaller fish called a scarlet wrasse.
Commercial fishers earn money by catching and selling large numbers of fish.Here，Japanese fishers haul in salmon.
Kinds of Fish