Which of the following organisms has a closed circulatory system?

A closed circulatory system is characterized by blood being contained within vessels, allowing for more efficient transport of nutrients, gases, and wastes compared to an open circulatory system, where blood flows freely within cavities. Among the options provided, mammals, including monkeys, possess a closed circulatory system. This system features a heart that pumps blood through a series of arteries, veins, and capillaries, ensuring that oxygenated and deoxygenated blood are kept separate and that the transport of materials is more effective.

In contrast, arthropods typically have an open circulatory system where blood (hemolymph) flows freely through the body cavity, bathing the organs directly. Mollusks can have either a closed or open circulatory system, but many like octopuses possess a closed system. Planarians, being flatworms, also have a more rudimentary system that does not classify as closed; they rely on diffusion for nutrient and gas exchange. Thus, the distinction of having a fully closed circulatory system applies specifically to monkeys among the options presented, showcasing the advanced complexity of their circulatory physiology.