The excretion process is concerned with the removal of waste products from the animal body. Dependent upon the process of excretion is the relatively stable internal environment of the organism, a concept known as homeostasis. Homeostasis means “staying the same.” It refers to a relatively stable internal environment, or a steady‐state equilibrium that exists internally in a healthy organism, despite changes in the external environment. The excretory system plays a major role in homeostasis.
Because one-celled organisms are in constant contact with their environment, they do not need excretory organs. However, multicellular organisms need a mechanism to carry waste products from cells to the external environment. Flatworms, such as planaria, have a series of excretory cells, called flame cells. Flame cells contain cilia that direct water and metabolic wastes to enter the cells and to pass into excretory canals. The excretory canals join with other canals to form excretory tubules. Fluid from the excretory tubules leaves the body through pores.
In earthworms, members of the phylum Annelida, the excretory system consists of structural units called nephridia. Each nephridium contains a ciliated tunnel that leads to a long, coiled tubule, which leads to a bladderlike sac (a primitive bladder). Fluid moves from the internal environment into the funnel. As fluid passes through the tubule, cells in the tubular lining absorb useful compounds such as glucose, amino acids, and salts. The remaining materials constitute metabolic waste, and they are passed into the bladderlike sac. The sac later opens through a pore in the earthworm's skin where the waste products are discharged.
Insects have a series of tubules for excretion called Malpighian tubules. Fluid enters at the upper end of the tubules and passes down their entire length. The cells in the tubular walls reabsorb precise amounts of water, salts, and other materials to maintain delicate balance within the insect tissues. The tubules eventually lead to an insect's intestine where waste products are removed.