based on the lab experiment on surface area to volume ratio need help please?

planaria(free-living  flatworms of the phylum platyhelminthes)have no true circulatory or respiratory systems. They do not reach sizes greater than approximately 8mm long. Humans obviously, grow to much larger sizes (volumes and masses),and possess complex circulatory and respiratory systems. Explain why planaria are limited to a small size, while many other animals (humans, for example)are able to support a far more massive and volumous body plan. Answer specifically, and base your reasons on the results of this lab excercise.  please help need some feedback....