Crab (páng xiè): Animal kingdom, arthropod, Crustacea, Soft Crustacea, soft armor, Decapoda, Gastropoda, Anura. It is a crustacean. Its body is protected by a hard shell and breathes through its gills. Taxonomically, it is the same kind of animal as shrimp, lobster and hermit crab. Most kinds of crabs live in the ocean or offshore areas, and some live in fresh water or land. They rely on the mother crab to give birth to small crabs, and every time the mother crab will lay a large number of eggs, the number can reach more than one million. Crabs rely on the geomagnetic field to judge their own direction. China has a long history of eating crabs, which is recorded in Yi Zhou Shu by Wang Huijie and Zhou Li Tian Guan's bumper harvest man.

The amount of genetic material of this kind of organism is complex and varies greatly, often as many as 100 pairs. Common crabs are hairy crabs (river crabs, hairy crabs, blue crabs) and swimming crabs. China is rich in crab resources, among which the hairy crabs produced in Gucheng Lake, Taihu Lake, gaoyou lake, Yangcheng Lake and Xiefu in the lower reaches of the Yangtze River are the top grade.

Although the crab is small, it covers everything. After removing the hard shell of the crab, we can find that the body part of the crab is protected by a shell. These shield-like shells are called crustaceans by biologists.

Crabs are symmetrical and can be divided into forehead area, eye area, heart area, liver area, stomach area, intestine area and gill area. The two sides of a crab are connected by appendages. The appendages of the head are called tentacles, which have the functions of touch and smell. Some appendages have the function of mouth, which is used to tear food and send it into the mouth.

Crab chest has five pairs of appendages, which are called chest feet. A pair of appendages at the front end are equipped with powerful claws, which can be used for foraging. The other four pairs of appendages are crab feet. Crabs rely on these four pairs of appendages to walk and move. The way they walk is unique and interesting, and most of them walk sideways rather than straight. Except monks and crabs, they all walk straight.

Crabs rely on the geomagnetic field to judge their own direction. In the long years after the formation of the earth, the geomagnetic north and south poles reversed many times. Many creatures are at a loss and even become extinct because of the reverse transfer of the earth's magnetic poles. Crab is an ancient migratory animal. Its inner ear has a small directional magnet, which is very sensitive to geomagnetism. Due to the reversal of the geomagnetic field, the small magnet in the crab lost its original direction. In order to survive the reversal of the geomagnetic field, the crab adopts the practice of "changing with the constant", simply walking sideways without advancing or retreating.

Crabs rely on the geomagnetic field to judge their own direction. In the long years after the formation of the earth, the geomagnetic north and south poles reversed many times. Many creatures are at a loss and even become extinct because of the reverse transfer of the earth's magnetic poles. Crab is an ancient migratory animal. Its inner ear has a small directional magnet, which is very sensitive to geomagnetism. Due to the reversal of the geomagnetic field, the small magnet in the crab lost its original direction. In order to survive the reversal of the geomagnetic field, the crab adopts the practice of "changing with the constant", simply walking sideways without advancing or retreating.

The crab's head and chest are indistinguishable in appearance, so it is called head and chest. The feet of this animal grow on both sides of the body. The first pair of pliers are not only tools for digging holes, but also weapons for defense and attack. The other four pairs are used for walking, called walking feet. Each foot consists of seven joints, and the joints can only move up and down. The chest width of most crabs is greater than the length, so when crawling, they can only bend their feet at one side, scratch the ground with their toes, and extend their feet at the other side. When the toes reach the distant ground, they begin to contract, while the originally bent feet immediately straighten and push the body to the opposite side. Because these pairs of feet are different in length, the crab actually walks forward. However, not all crabs can only run wild. For example, monks with long wrists and crabs who live in groups on the beach can run forward. Many spider crabs that live in seaweed can also climb vertically on seaweed.

Through experiments, it is found that each limb in the crab (where the muscle beam passes) has two bone eyes (that is, two muscles) connected. Moreover, the bending direction of the root joints of its limbs is the dorsal direction, so when the muscles contract, they will pull the limbs to move in the dorsal direction, so the crab will move sideways.

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