Is there anything special about daphnia?
As far as I know organisms require a heart to act as a pump and enable efficient transport of substances like nutrients and respiratory gases for cells.
Also, what i can figure is a heart and circulatory system in required in this case, cause maybe the surface are to volume ratio of daphnia is not high enough to allow exchange of substances on its surface. So, an internal system is required
Maybe I can help more...lemme know
