In humans, solid waste is expelled as faeces from the digestive tract. Cellular, or metabolic, waste is processed by the kidneys, liver and lungs and expelled by urination, by exhalation and through the skin.
Earthworms have a digestive system and a semi-permeable skin, but they do not have organs similar to the kidney, liver or lungs. Instead, food moves is processed through a system of muscles and intestines and is expelled as waste through the anus.
The earthworm's digestive system is a muscular tube that extends the length of the animal, from the mouth to the anus. It contains several specialised areas: the pharynx, where food is ingested; the crop, where food is stored; the gizzard, where food is processed; and the intestine, where food is digested and nutrients are absorbed.
Solid Waste Removal
Earthworms eat organic material such as leaf debris, grass clippings and seeds. The mouth is simply an opening; it has no teeth and plays no part in breaking down the food. Food is sucked into the mouth through muscular action of the pharynx and passes through the oesophagus into a storage area called the crop. From there it is moved into the muscular gizzard, which also contains hard soil particles. The muscular contractions of the gizzard and the abrasion of the particles grind down the food. Digestion and the absorption of nutrients takes place in the intestine, and the undigested solid waste is expelled through the anus.
Metabolic Waste Organs
The earthworm's body takes the form of a linear series of segments, or compartments. Almost every segment of the earthworm's body contains a pair of tightly coiled tubes called nephridia. These tubes are very similar in form and function to the nephrons of the human kidney. The tubes are in close contact with body fluids and are well-supplied with blood capillaries. They are specialised to absorb metabolic waste products.
Metabolic Waste Excretion
Each nephridium tubule absorbs and collects waste products from the cellular fluid and blood that surrounds it. The tube is lined internally with cilia--tiny hairs that move together to provide directional motion to the contents of the tube. The wastes travel through the coiled tube into a bladder at the end, which expels the waste as urine through an opening.
Definition of Excretion
Any further reading on the subject should note that the biological definition of excretion is different from the lay definition. Biologically speaking, the term "excretion" refers only to the removal of metabolic waste from an organism; it does not include the expulsion of solid, undigested waste. However, most people do consider solid waste removal as part of the overall process.