Final answer:
An open circulatory system is less efficient at transporting vital substances, leading to slower metabolic rates and limiting the potential size of animals. Closed circulatory systems, with their network of vessels, allow for larger body sizes and greater activity levels.
Step-by-step explanation:
An open circulatory system constrains the size of animals because it is less efficient at transporting oxygen and nutrients as compared to a closed circulatory system. Oxygen and nutrient transport is crucial for maintaining a high metabolic rate, which is necessary for the survival of larger animals. In an open circulatory system, hemolymph (a mixture of blood and interstitial fluid) is pumped through vessels by a simple heart and spills into body cavities where tissues are bathed in this fluid. This method is inherently less efficient due to the lack of dedicated blood vessels, which leads to slower transport of vital substances and a limited capacity to support large bodies. Consequently, animals with open circulatory systems, such as insects, are usually smaller in size. Closed circulatory systems, in contrast, have a network of vessels that efficiently deliver oxygen, nutrients, and remove waste products, thereby enabling animals to grow larger and be more active. The evolution from open to closed circulatory systems has allowed vertebrates to support greater body sizes and more complex activities.