Respiratory Systems in Other Animals
. The need to take in oxygen and expel carbon dioxide is almost universal among organisms. The movement of these gases between an organism and its environment, called gas exchange, is accomplished in a variety of ways by different organisms. In one-celled aquatic organisms, such as protozoans, and in seaweeds, sponges, jellyfish, and other aquatic organisms that are only a few cell layers thick, oxygen and carbon dioxide diffuse directly between the water and cells. Diffusion works for these simple organisms because all cells of the organism are within a few millimeters of an oxygen source. Animals with many cell layers cannot rely on diffusion because cells several layers deep in the body would die before oxygen reached them. As a result, for gas exchange, more-complex animals require special respiratory organs, such as gills or lungs, in combination with circulatory structures, such as blood, blood vessels, and a heart. The earliest development of these gas exchange structures is see