An arthropod is an invertebrate animal of the large phylum Arthropoda, such as an insect, spider, or crustacean. This is that largest as well as most diverse group in the animal kingdom. We are going to focus mainly on the insects, since they are more well-known. The organisms that belong to this phylum make up more than 75% of all life on Earth. Arthropods are also found all over the world in a wide variety of environments, you can find them deep in the oceans or in the hottest deserts. Arthropods include four major classes, Chelicerates (spiders, ticks, scorpions, horseshoe crabs, and mites), Crustaceans (lobsters, crabs, and shrimp), Uniramians (millipedes, centipedes, and insects), and the now extinct Trilobites. Over 800,000 species of arthropods have been identified; however, some scientists are speculating that there are tens of millions of species in this specific phylum that are not yet discovered. When it comes to knowing whether or not an organisms is classified as an arthropod we must see if they possess these characteristics:Segmented body: This means that their bodies are split up into more than one part, internally or externally. Exoskeleton: All arthropods are invertebrates, this means that their bodies don’t have internal bones for support. To protect them instead, since they do not possess an internal skeletal system they have a hard exoskeleton which is made out of chitin, a mix of lipids, carbohydrates, as well as protein. The exoskeleton protects their bodies like armor. However as arthropods grow they shed or in some cases molt their exoskeletons; However, in order to do this they firstly must produce new as well as softer exoskeletons under the old ones. Then once they’re hardened the old covering cracks and sheds. During this time arthropods are extremely vulnerable and usually hide until the new exoskeletons harden.Jointed limbs: Arthropod originally comes from the Greek word “arthro” meaning joint, and “pod” meaning foot. The jointed limbs that all arthropods have are attached to their hard exoskeletons. This then allows for flexibility and movement. Although the joints generally bend in one direction they still allow for predatory and defensive actions. Bilateral Symmetry: When looking at an arthropod’s body it can be divided vertically into two mirror images. This is known as bilateral symmetry. Arthropods also share this characteristic with other organisms one being humans. Open Circulatory System: Arthropods also have an open circulatory system. Instead of having a closed circulatory system of interconnected veins and capillaries like humans do, an arthropod’s blood is pumped through open spaces called sinuses in order to reach tissues.