Final answer:
Cnidarians do not have a circulatory system like other animals. They rely on diffusion for gas exchange and nutrient movement. The lack of a circulatory system limits their body thickness and requires a non-living layer called the mesoglea.
Step-by-step explanation:
As illustrated in Figure 40.4a, cnidarians such as corals, sea fans, and sea anemones do not have a circulatory system like other animals. Instead, they rely on diffusion for gas exchange and the movement of nutrients. The lack of a circulatory system limits the thickness of their bodies and necessitates a non-living layer called the mesoglea. While some cnidarians have canals to help distribute nutrients and gases, they do not have a true circulatory system like organisms with closed or open circulatory systems.