Today, arthropods make up 80% of all animals, which includes insects, arachnids, and crustaceans. An arthropod has a segmented body covered by an exoskeleton, which protects the body and provides places for muscle attachment. All arthropods have a heart. All species in this phylum have an open circulatory system, in which blood flows freely throughout the entire body to give oxygen while carbon dioxide is disposed. Also, blood in the circulatory system makes direct contact with tissues and organs within the body. However, the blood in insects does not carry oxygen.