Why do bryozoans lack circulatory and respiratory systems?

Bryozoans, also known as moss animals, do not possess specialized circulatory or respiratory systems because their small body size allows them to meet their physiological needs through simple diffusion.1

Individual zooids are typically no longer than one millimeter in length. At this scale, the distance between the animal's internal tissues and the surrounding aquatic environment is sufficiently small that oxygen can be absorbed and metabolic waste products can be expelled directly across the body wall. Because these organisms are so small, they do not require the complex transport mechanisms—such as blood vessels or specialized respiratory organs—that larger, more complex animals rely on to move gases and nutrients throughout their bodies.21

While they lack these specific systems, bryozoans are still highly functional; they utilize a lophophore to collect food and possess a digestive tract to process nutrients, which are then distributed throughout the colony via interconnecting pores or tissues.13