(2) The structure that can generate forward power and control forward direction is a 4-caudal fin.
(3) In the process of observing crucian carp, the body of crucian carp is spindle-shaped, which is beneficial to reduce the water resistance during swimming.
(4) The color of the back of crucian carp is dark gray-black, which is consistent with the color of the bottom of the water, and the enemy is not easy to find; The belly is white, which is the same color as the background from the depths of the water and is not easy to be found by the enemy. Therefore, it is a protective color, which can reduce the chance of natural enemies killing themselves.
So the answer is:
(1) pectoral fin; Abdominal fin; Gluteal fin; Tail fin; Dorsal fin;
(2)4;
(3) shuttle; Reduce water resistance when swimming;
(4) Dark gray-black, white.