Briefly explain the following:
i. Flame cells.
ii. Nephridia.
i. Flame cells - They are also known as protonephridia. These are the excretory structures found in platyhelminthes, rotifers and some annelids. They are primary concerned with ionic and fluid volume regulation. 

ii. Nephridia - are tubular excretory strauctures found in earthworms and other annelids. These help to remove the nitrogenous wastes and maintain fluid and ionic balance.