Single-celled organisms serve all of their needs through direct exchanges with the environment. Such organisms are found mostly in aquatic or very moist terrestrial environments. Similarly, many multicellular aquatic organisms are small or thin enough that all of their cells are close to the external environment. Such species may not have a circulatory system because nutrients, respiratory gases, and wastes can diffuse directly between the cells of their bodies and the environment.

The cells of some larger aquatic multicellular animals without a circulatory system are served by highly branched central cavities called gastrovascular systems that bring the external environment into the animal. All the cells of a sponge are in contact with, or very close to, the water that surrounds the animal and circulates through its central cavity (see Figure 48.1B). Very small animals without a circulatory system can maintain high levels of metabolic activity, but larger animals without a circulatory system such as sponges, jellyfishes, and flatworms tend to be inactive, slow, or even sedentary. Large, active animals require circulatory systems.