It is then ready to be pumped from the heart. This type of circulatory system is called an open circulatory system. Digestive waste leaves through the anus. Nitrogen waste is extracted from the blood by green glands. These are located at the anterior of the animal, and pores through which the nitrogen waste leaves are close to the eyes and antennae.
Gas exchange occurs in the gills.
Blood carries carbon dioxide waste to the gills. These are large feathery structures with a great deal of surface area. They are located outside the body wall but inside the skeleton. As water flows by the gill, carbon dioxide is released into the water by diffusion from the blood. Oxygen from the water flowing past the gills diffuses into the blood within the gills.