An organism living in a completely aquatic environment has several advantages: their body is supported inside and out, the current of water constantly bathes their cells and brings nutrients and oxygen while carrying away wastes, and rapid changes in temperature were prevented. When organisms moved out of an aquatic environment into a terrestrial environment, they still needed these functions performed. Many of our organs/systems evolved to solve this problem and take on a specific function once provided by free water in an aquatic environment.
1. Use one of the organ systems of any organism to create an analogy between it and the functions water once performed in an aquatic environment.