In protozoans, porifers and coelenterates the specialised excretory structures are absent. The excretion in these animals occurs through the general body surface. The contractile vacuoles present in freshwater protozoans are the osmoregulatory structures and regulate osmotic concentration (osmolarity) of the body. Though some ammonia is also lost through contractile vacuoles, but they can not be considered as excretory structures. In Platyhelminthes (flatworms) the excretory structures are Protonephridia or Solenocytes. These structures are also present in Rotifers, Cephalochordates (Amphioxus). and some annelids. The tubules of protonephridia in flatworms terminate into Flame cells. These cells contain a tuft of cilia which project into the tubule and beat like flickering (candle) flame, hence…