Solenocytes or flame cells or protonephridia are characteristic of Platyhelminthes. They are modified mesenchymal cells. Each flame cell has an intracellular lumen or cavity in which cilia are hang. The lumen of flame cell is continuous with a microscopic capillary duct. The excretory system of annelids consist of coiled tubes called nephridia. They are three types pharyngeal, integumentary and septal. The excretory organs in Mollusca are metanephridia. Excretory organs are absent in Echinodermata.