Question

Some animals use diffusion instead of a circulatory system. Examples include:

a) birds and jellyfish
b) flatworms and arthropods
c) mollusks and jellyfish
d) None of the above

Answer 1

Simple animals like sponges and jellyfish use diffusion instead of a circulatory system. More complex organisms, such as arthropods and mollusks, have open circulatory systems, while vertebrates possess closed circulatory systems.

Step-by-step explanation:

The correct answer to the student's question is that some animals use diffusion instead of a circulatory system. Specifically, simple animals such as sponges and jellyfish do not possess a circulatory system. These organisms, which may consist of a single cell layer or just a few cell layers, exchange gases, nutrients, and wastes primarily by diffusion. This is possible because their body structures allow nearly all of their cells to be in direct contact with the external environment or with the internal gastrovascular compartment that is also exposed to the environment.

For more complex organisms, such as arthropods and mollusks, an open circulatory system is more common, where a fluid known as hemolymph is pumped through their bodies. The most active mollusks, like squids, and all vertebrates, including mammals and birds, have developed closed circulatory systems that are more efficient for distributing nutrients and oxygen as well as for removing waste throughout their more complex body structures.