- Compare and contrast the way in which planaria and annelids handle waste products
As multi-cellular systems evolved to have organ systems that divided the metabolic needs of the body, individual organs evolved to perform the excretory function. Excretory cells known as flame cells developed in flatworms, while nephridia developed as excretory cells in annelids.
Flame Cells of Planaria
Planaria are flatworms that live in fresh water. Their excretory system consists of two tubules connected to a highly-branched duct system that leads to pores located all along the sides of the body. The filtrate is secreted through these pores. The cells in the tubules are called flame cells (or protonephridia) because they have a cluster of cilia that looks like a flickering flame when viewed under the microscope. Flame cells function like a kidney, removing waste materials through filtration. The cilia propel waste matter down the tubules and out of the body through excretory pores that open on the body surface; cilia also draw water from the interstitial fluid, allowing for filtration. After excretion, any useful metabolites are reabsorbed by the cell. Flame cells are found in freshwater invertebrates, such as flatworms, including parasitic tapeworms and free-living planaria.
Flame cells and nephridia: In the excretory system of the (a) planaria, cilia of flame cells propel waste through a tubule formed by a tube cell. In (b) annelids, nephridia filter fluid from the body cavity.
Nephridia of Worms
Earthworms (annelids) and some other invertebrates, such as arthropods and mollusks, have slightly-more-evolved excretory structures called nephridia. A pair of nephridia is present on each segment of the earthworm. They are similar to flame cells in that they have tubules with cilia and function like a kidney to remove wastes, but they often open to the exterior of the organism. The ciliated tubules filter fluid from the body cavity and carry waste, including excess ions, through openings called nephrostomes. From the nephrostomes, excretion occurs through a pore called the nephridiopore. A nephridium is more evolved than a flame cell in that it has a system for reabsorption of some useful waste products, such as metabolites and ions, by a capillary network before excretion (unlike planaria that can only reabsorb useful metabolites after excretion).
- Nephridia are more evolved than flame cells because they can reabsorb useful metabolites before excretion of waste.
- Both nephridia and flame cells are ciliated tubules that filter fluids in the cell to remove waste.
- Flame cells are connected to a duct system of pores to expel wastes, while nephridia often are open to the exterior of the organism.
- flame cell: a specialized excretory cell found in the simplest freshwater invertebrates
- nephridium: a tubular excretory organ in some invertebrates
- nephridiopore: the external opening of a nephridium, where waste is excreted from the cell
- nephrostome: the funnel-shaped opening of a nephridium into the body cavity