The

excretory system The excretory system is a passive biological system that removes excess, unnecessary materials from the body fluids of an organism, so as to help maintain internal chemical homeostasis and prevent damage to the body. The dual function of excreto ...

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

s removes nitrogenous waste and maintains the internal water balance of these creatures, commonly referred to as

snail A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gastro ...

s and

slug Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusc. The word ''slug'' is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a smal ...

s. The primary organ of excretion is a

nephridium The nephridium (plural ''nephridia'') is an invertebrate organ, found in pairs and performing a function similar to the vertebrate kidneys (which originated from the chordate nephridia). Nephridia remove metabolic wastes from an animal's body. Neph ...

Structure

The most primitive gastropods retain two nephridia, but in the great majority of species, the right nephridium has been lost, leaving a single excretory organ, located in the anterior part of the

viscera In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a ...

l mass. The nephridium projects into the main venous sinus in the animal's foot. The circulatory fluid of gastropods, known as

haemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...

directly bathes the tissues, where it supplies them with oxygen and absorbs carbon dioxide and nitrogenous waste, a necessary waste product of

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...

. From the arterial sinuses bathing the tissues, it drains into the venous sinus, and thus flows past the nephridium. The main body cavity of most aquatic gastropods also includes pericardial glands, often located above the heart. These secrete waste into the haemolymph, prior to further filtration in the nephridium. Pulmonates lack these glands, so that the nephridium is the only major organ of excretion. In some gastropods, the nephridium opens directly into the sinus, but more usually, there is a small duct, referred to as the renopericardial canal. In aquatic gastropods, the nephridium is drained by a

ureter The ureters are tubes made of smooth muscle that propel urine from the kidneys to the urinary bladder. In a human adult, the ureters are usually long and around in diameter. The ureter is lined by urothelial cells, a type of transitional epit ...

that opens near the rear of the mantle cavity. This allows the flow of water through the cavity to flush out the excreta. Terrestrial

pulmonate Pulmonata or pulmonates, is an informal group (previously an order, and before that a subclass) of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills. The group includ ...

s instead have a much longer ureter, that opens near the

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

. In addition to the pericardial glands and nephridium, excretory cells are also present in the digestive glands opening into the

stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...

. These glands have a metabolic function, somewhat similar to that of the vertebrate

liver The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...

, and excrete waste products directly into the digestive system, where it is voided with the

faeces Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...

Water balance

In aquatic gastropods, the waste product is invariably

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...

, which rapidly dissolves in the surrounding water. In the case of freshwater species, the nephridium also resorbs a significant amount of salt in order to prevent its loss through

osmosis Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region of ...

into the surrounding water. Terrestrial species instead excrete insoluble

uric acid Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates, such as ammonium acid urate. Uric acid is a product of the metabolic breakdown of ...

, which allows them to maintain their internal water balance. Even so, most species require a somewhat humid environment, and secrete a considerable amount of water in their slime trail. Those few species that dwell in arid environments typically

hibernate Hibernation is a state of minimal activity and metabolic depression undergone by some animal species. Hibernation is a seasonal heterothermy characterized by low body-temperature, slow breathing and heart-rate, and low metabolic rate. It most ...

aestivate Aestivation ( la, aestas (summer); also spelled estivation in American English) is a state of animal dormancy, similar to hibernation, although taking place in the summer rather than the winter. Aestivation is characterized by inactivity and a ...

during dry periods to preserve moisture.

References

External links

{{Gastropod anatomy Gastropod anatomy Organ systems