Because though there is a lot of available water, an ever shrinking 3% is freshwater, which comes from groundwater and streams. The ocean is salinated and in order for it to be fit to be used by humans for domestic needs or for drinking, it needs to undergo a huge desalination process which costs lots of money and time.
Answer: no body cavity—acoelomates; body cavity not completely lined with mesoderm—pseudocoelomates; body cavity completely lined with mesoderm—coelomates
