The Blue Crab is a marine arthropod. It is part of over 5000 different species of crab. 4500 species are known as true crab because the other 500 species consists of hermit crabs.
The respiratory system of the Blue Crab consists of an exoskeleton with spike-like protrusions that act as small gills that also work on land. These gill-like protrusions then connect to ribbon-like veins that connect to the heart and muscles to allow functionality. Although they vary in structure and location, the gills are always outgrowths of the skin and are therefore covered by the exoskeleton, which is thin in this area and not a barrier to the exchange of gases.