Why do larger animals, such as a bullfrog, require a circulatory system, while animals such as a planarian do not?
Each cell in a bullfrog requires more oxygen and nutrients than each cell in a planarian.
The cells of a bullfrog are many times larger than the cells of a planarian.
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.
None of the choices is correct.
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.