Crabs Anatomy| Interesting Facts

Crabs anatomy is not much complicated, Crabs are fascinating creatures and have body parts of their own features. Each type of crab has different characteristics. The crab’s body is covered with a hard shell made up of chitin.

crab external anatomy

Crabs External anatomy:

Crabs have many body parts, as follows:

  • Claws and chelas
  • Abdomen
  • Carpus
  • Eyes
  • Dactyl
  • Cephalothorax
  • Antenna 
  • Swimming legs
  • Walking legs 
  • Merus
  • Propdrus

Claws and chelas

In fact Crabs have three pairs of legs which end in large claws. The crabs utilize their first pairs of feet to capture prey, eat food, and defend themselves.

The following four pairs are used for walking. The last pair of legs are called swimming legs which are used for moving through water.

Crabs mostly walk sideways, and this way is more efficient. This is because their legs are present on the sides.


The abdomen of the crabs is unprotected and a delicate part of the body. It is located under the cephalothorax, including the seven segments.

Cephalothorax| Crabs Anatomy

The cephalothorax includes the head and thorax. The hard covering of the head and thorax is called the carapace. It saves the delicate and vital parts of the crabs.


Crabs have two pairs of antennae. Crabs use their antennae to gather information. They can identify the water’s vibrations around the crab. The short antennas help taste and smell when touching and eating food. They are called chemoreceptors.


The head of the crab has many appendages.

  • The first and second eyes and antenna
  • All mouthparts include the maxilla, maxillipeds


Crab’s chest is present on the upper part of the body. It is thicker than the shell and connects to five pairs of appendages.

Mouthparts| Crabs anatomy

Mouthparts| Crabs Anatomy

Maxillipeds are mouthparts modified to bring food to the mouth. The first and second maxillipeds are smaller and help in taking food. They also help to bring water to the gills so that crabs can breathe while eating.

Maxillipeds remain constant as crabs take tiny morsels into the crabs mouth. Both are combined and to the head. Mandibles are designed with powerful muscles and are anterior to the mouthparts. The mandibles tear food parts into small pieces and move them side to side.


Carpus is a heavy-boned structure, more comprehensive than at the side.

Stalked Eyes

The Crab’s eyes are visual organs located on the ends of the eyestalk. Crabs can bend their eyes in the shell for safety. Their Eyes cannot focus on anything because they are sensitive.

The eyestalks release a hormone that is responsible for the balance of metabolism and the molting process.

Crabs’ internal anatomy:

Circulatory system:

An open circulatory system in crabs means their blood does not circulate in veins and arteries. 

The hemolymph cleans the internal parts when the crabs walk, and oxygen reaches them through the branchial chamber and the lungs. The heart pumps the hemolymph, which circulates within the body. When the heart eases up, the blood goes back to the core by pores called ostia. These pores replace the veins in the crab’s body.

Respiratory system| Crabs Anatomy

Actually, Crabs have different respiratory systems according to their habitat. Aquatic crabs breathe through their gills, while land crabs have lungs for respiration. Crabs can get oxygen from the water and air.

Underwater, the crabs use a second pair of Maxillae, allowing water to pass through their gills. On the land, crabs have particular adaptations and use articulating plates for breathing. It keeps their gills moist and prevents dryness.

The digestive system

The crab’s digestive system consists of three parts.

  • The foregut work in extracellular digestion
  • The midgut performs the primary digestion process
  • The hindgut starts from the midgut and ends at the anus.

As you know, crabs do not have teeth, so they cannot tear food into small pieces. The foregut of the crabs has gastric mills, which help further the digestion of the food. Like other crustaceans, crabs also have digestive glands which work as the liver and pancreas. It absorbs all the nutrients and works for digestion and storage. 

The nervous system| Crabs Anatomy

Certainly, Crab’s nervous system is condensed and consists of many ganglia. It has a brain that is connected to a nerve of the ganglia.

The brain is on the anterior end of the head, and the ventral nerve cord is spread to the legs. The crabs have three pairs of ganglia.

  • The protocerebrum
  • The deutocerebrum
  • The tritocerebrum

The central nerve cord is present along the belly and runs to the telson. It has small nerve clusters at every end of the segments.

The ventral nerve cord consists of several nerve cords.

  • The subesophageal ganglion is responsible for sense and stimulates mouthparts.
  • The thoracic ganglia have neurons that are important for the movement of the appendages.
  • The abdominal ganglia have reproductive organs and an excretion system.
  • The last ganglia provides information to the body.

Reproductive system

In fact the male crab’s reproductive system has testes. These testes are present near the heart. Actually, these testes have seminiferous tubules with a layer of epithelial cells that transfers spermatozoa.

The females have pairs of ovaries in which eggs are produced. The ovaries are dorsal to the digestive tract and extend to the body cavity of the cephalothorax.

The ovaries alter from a light pink immature ovary to an orange mature ovary.

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