Circulatory System of Cnidarians
Cnidarians lack a circulatory system because they simply do not need one. All the cells of cnidarians are in contact with the water, which has all of the oxygen and nutrients the simple animal needs. The use for a circulatory system is to transfer oxygen and nutrients to all the cells of the body, and take away the waste. A circulatory system is just not necessary for cnidarians because there are no cells that need oxygen and nutrients delivered to them.