Why do larger animals, such as a bullfrog, require a circulatory system, while animals such as a planarian do not?

• A. Each cell in a bullfrog requires more oxygen and nutrients than each cell in a planarian.
• B. The cells of a bullfrog are many times larger than the cells of a planarian.
• C. Gases do not diffuse through bone. Therefore, any animal with bones, such as a bullfrog, has to have a circulatory system. Any animal without bones, such as a planarian, does not have a circulatory system.
• D. None of the choices is correct.
• E. Diffusion is inadequate to move gases through the body surface and into all parts of a larger animal body. However, diffusion alone is sufficient to move gases in and out of the thin body of a planarian.

Answer: (E)

Verified step by step guidance

1. Understand the concept of diffusion: Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. It is a passive process that does not require energy.
2. Consider the size and complexity of the organism: Larger animals, like bullfrogs, have a greater volume and more complex body structures compared to smaller, simpler organisms like planarians.
3. Analyze the limitations of diffusion in larger organisms: In larger animals, diffusion alone is not sufficient to transport oxygen and nutrients to all cells efficiently due to the greater distance and complexity of tissues.
4. Recognize the role of the circulatory system: Larger animals have developed circulatory systems to efficiently transport gases, nutrients, and waste products throughout their bodies, overcoming the limitations of diffusion.
5. Compare with simpler organisms: In smaller organisms like planarians, their thin and flat body structure allows diffusion to be sufficient for gas exchange and nutrient distribution, eliminating the need for a circulatory system.