The hard carapace on the crab can protect the crab from natural enemies, but the carapace will not expand with the growth of the body. Therefore, the growth of crabs is intermittent, that is, after a period of time, the body will continue to grow after the old shell is shed. The largest crab on the earth is the spider crab, whose feet are 3.7 meters wide, and the smallest crab is the bean crab, which is less than half a centimeter in diameter.

Although the crab is small, it has all the internal organs. 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 carapace by biologists. Crab body is symmetrical, which can be divided into forehead area, eye area, heart area, liver area, stomach area, intestine area and gill area. Both sides of the crab's body are connected by appendage. 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.

The crab chest has five pairs of appendages, which are called pectoral feet. A pair of appendages at the front are equipped with strong claws, which can be used for foraging. The other four pairs of appendages are the feet of crabs. The crabs rely on these four pairs of appendages to walk and move. They walk in a unique and interesting way, and most of them walk sideways instead of straight ahead. Except for monks and crabs, they walk straight.

Crabs spend most of their time looking for food. They are not picky eaters, and they can eat anything that their claws can get. Small fish and shrimp are their favorite, but some crabs eat seaweed, even animal carcasses or plants.

crabs eat other animals, and other animals may also eat crabs. For example, humans regard crabs as delicacies. And waterfowl eat crabs! Some fish like to eat crab feet just like humans. When young and underage crabs float in the sea in groups, they may be wolfed down by other marine life, so crabs have to lay a lot of eggs when laying eggs.