Excretion Systems

Flame Cells of Planaria and Nephridia of Worms

As multicellular systems evolved to have organ systems that divided the metabolic needs of the body, individual organs evolved to perform the excretory function. Planaria are flatworms that live in freshwater. Their excretory system consists of two tubules connected to a highly branched duct system. 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, as illustrated in Figurea. 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. Any valuable metabolites are recovered by reabsorption. Flame cells are found in flatworms, including parasitic tapeworms and free-living planaria. They also maintain the organism’s osmotic balance.

Illustration A shows a flame cell, which is bulb-shaped with cilia projecting from one end. The cilia form a point, like the tip of a paintbrush, inside as wide opening at the end of a tube cell. The tube cell narrows into a tubule, then widens into a body where the nucleus is located. The tubule continues past the cell body. Illustration B shows a cross section of an earthworm, which is segmented with walls separating each segment. The trumpet-like opening of a nephridium sticks out of the wall. Cilia surround the opening. Beyond the wall, the nephridium forms a tube that winds down to the ventral surface, where it connects with an opening to the exterior. Just above the opening the tube widens into a bladder.
In the excretory system of the (a) planaria, cilia of flame cells propel waste through a tubule formed by a tube cell. Tubules are connected into branched structures that lead to pores located all along the sides of the body. The filtrate is secreted through these pores. In (b) annelids such as earthworms, nephridia filter fluid from the coelom, or body cavity. Beating cilia at the opening of the nephridium draw water from the coelom into a tubule. As the filtrate passes down the tubules, nutrients and other solutes are reabsorbed by capillaries. Filtered fluid containing nitrogenous and other wastes is stored in a bladder and then secreted through a pore in the side of the body.

Earthworms (annelids) have slightly more evolved excretory structures called nephridia, illustrated in Figureb. A pair of nephridia is present on each segment of the earthworm. They are similar to flame cells in that they have a tubule with cilia. Excretion occurs through a pore called the nephridiopore. They are more evolved than the flame cells in that they have a system for tubular reabsorption by a capillary network before excretion.