Answer:
Open circulatory systems are advantageous to some animals because they use less metabolic energy.
Explanation:
Open circulatory systems are also known as lagoon circulatory systems. In this systems the blood is not necessarily always inside the blood vessels; It leaves the heart and will directly irrigate the organs and then return by other means or mechanisms.
So that it can be understood more easily, it is necessary to think about this system acting in organisms much smaller and less complex than mammals or any other vertebrate of greater size. As this animals doesn't have a great size, they need to use less metabolic energy than more developed organisms, so this system allows them to save metabolic energy to be used in other living functions.
I think that it is most visible in water because it separates from the water and stands out.
Well you fill a cup up with water. Then drink. Nuf Said
Http://spaceplace.nasa.gov/seasons/en/
