The body structure of an animal always adapts to its function. According to this basic viewpoint of biology, analyze and answer the following questions. Fish can live in water.

(1) gill (2) water evaporation (3) air sac (4) joint

Test analysis: (1) Fish has scales on its body surface, which can secrete mucus and reduce water resistance. The respiratory organ is gill, which absorbs dissolved oxygen in water, swims with fins and swims forward by swinging its tail and trunk from side to side;

(2) Insects have a hard exoskeleton, whose main function is to protect and support the internal soft organs and prevent the evaporation and loss of water in the body, which is one of the characteristics of adapting to land life;

(3) The air sac of birds is connected with the lungs, and its main function is to store air and assist breathing. Even if the inhaled air passes through the lungs twice, it can ensure the lungs to fully exchange gas, assist the lungs to complete double breathing, and provide sufficient oxygen for flight. At the same time, the airbag also has the functions of reducing the specific gravity of the body, emitting heat, regulating body temperature and reducing the friction between organs.

(4) In human body, the stimulation of skeletal muscle comes from the excitement of nerve. After being stimulated, skeletal muscles contract and pull the bones attached around the joints, thus making the human body produce various actions.