(1) Fish live in water and are suitable for aquatic organisms. The back of crucian carp is dark gray-black, which is consistent with the color of the bottom of the water and is not easy to be found by the enemy. The ventral surface of crucian carp is white, which is the same as the background color from the depth of water and is not easy to be found by the enemy.

(2) Crucian carp is spindle-shaped, which is beneficial to reduce water resistance and adapt to life in water.

(3) There is a lateral line on each side of the crucian carp, which is connected with the nerve and is mainly used to determine the direction and sense the water flow. Therefore, when observing the external morphology of crucian carp, it is found that there are two sensory organs on both sides of the body of crucian carp, which are called lateral lines.

(4) Steps of dissecting crucian carp: 1. Hold the head of the crucian carp with the left hand, so that the ventral surface of the crucian carp faces upward, and cut forward with the right hand from the anus until it reaches the lower edge of the gill cover (the first cut). 2. Place the crucian carp on its side in the dissecting tray so that its left side is upward, lift the left body wall with tweezers, and cut it back from the anus with dissecting scissors. Cut forward along the spine to the rear edge of the gill cover (second cut). 3. Cut forward along the trailing edge of gill cover to the front of pectoral fin (third cut). Then, take away the cut left trunk wall. 4. Cut off the left branchial operculum (fourth knife). Therefore, when dissecting crucian carp, the left body wall is cut.

(5) There are swim bladders in crucian carp, whose main function is to regulate the body proportion. With the cooperation of fins, the swim bladder can make the fish stay in different water layers. Therefore, observing the internal structure of crucian carp, it is found that there is a white bubble structure on the back of the body cavity, which can adjust the specific gravity of the body and make crucian carp stay in different water layers. Its name is swim bladder.

So the answer is:

( 1)

(2) Small (or reduced)

(3) lateral line

(4) Left

(5) swim bladder