Becuase they were fully aquatic unlike tetrapods today
Flash floods are population independent, and are therefore NOT dependent upon population density
They are useful because of the way they process and digest food. They apply their enzymes externally and then absorb smaller mollecules. This process releases things like CO2 or creates alcohols, and this has historically been used in making drinks like beer or foods like bread and cheese.
Always been taught that the vacuole holds the cell's water. I don't know about the other things <span />
