Final answer:
Cnidarians rely on diffusion through their gastrovascular cavity and do not have a circulatory system, so they do not exhibit single circulation of blood flow like some vertebrates do.
Step-by-step explanation:
Cnidarians, such as jellies and coral, do not have a circulatory system like humans; they do not have the concept of single or double circulation as seen in vertebrates. Oxygen and carbon dioxide exchange in cnidarians occurs by diffusion through the gastrovascular cavity and the external environment. The lack of a specialized circulatory system means that gases, nutrients, and waste simply diffuse into the surrounding water or gastrovascular cavity. This simplicity limits body thickness and demands the presence of mesoglea (a non-living layer) between the two layers of cells.
Organisms with more complex needs, like vertebrates, evolved more advanced circulatory systems. Cnidarians, in contrast, maintain their simple structure and rely on their aqueous environment to mediate the exchange of substances. The Anthozoa and Medusozoa, divisions of the Cnidaria phylum, exhibit these characteristics. Examples of Anthozoa include corals and sea anemones, whereas Medusozoa includes various jellyfish classes.