**Arthropods** are a group of animal species that have segmented bodies, a tough external exoskeleton that is used for protection and support, and joint appendages in the form of antennae, and legs. Examples of this species are insects, crabs, and spiders.

One of the main distinctions of **arthropods** from other animals is the presence of their segmented body and the tough exoskeleton covering. In fact, 500 million years ago, the first arthropod species found in the marine environment contains numerous body segments and a walking leg. However, through the course of evolution, and natural selection, these body segments are now reduced in number — only specializing in those important body processes such as feeding and locomotion.

When an arthropod grows, the exoskeleton does not grow with it. Instead, the exoskeleton sheds off through a process called molting. Once the entire exoskeleton is gone, the glands will make a larger one to take its place.

**Arthropod’s endocrine system controls molting process**. When the animal is growing out of the exoskeleton, the endocrine system releases hormones that digest the inner layer of the exoskeleton and signals the production of new exoskeleton.

Arthropods have an open circulatory system that contains a heart that pumps blood into arteries that lead to an open cavity. Then, the blood from the cavity goes back into a large sinus that surrounds the heart.

On the other hand, excretion takes place in a saclike organ called Malpighian tubules. Metabolic wastes are extracted from the blood using this organ. Once the wastes are collected, they are diffused into the water.

The processes of excretion and circulation both involve the transport of blood. Since blood is being transported by the circulatory system, the work of the excretory system is to filter the wastes found in the blood.

Unlike other animal cells, the cell walls of arthropods contain chitin. This is the primary component found in their exoskeleton. When chitin is combined with other proteins, the exoskeleton of arthropods can evolve into different forms. This allowed arthropods to have different appendages such as wings, flippers, claws, and antennae. This is why some insects are able to crawl, swim, or fly.