(b) : A nephron is a unit of structure and function in a kidney. A kidney contains about a million nephrons, each approximately 3cm long. A nephron is a long tubule differentiated into four regions haying different \/ anatomical features and physiological role : Bowman's capsule, proximal convoluted tubule (PCT), loop of Henle, and distal convoluted tubule (DCT). The latter opens into one of the collecting ducts. Nephridia are the excretory organs of annelids.