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Structure and origin of the excretory system of flat, round and annelids.
The excretory system consists of protonephridia. These are branching canals ending in so-called flame cells—hollow cells with bundles of constantly moving cilia.
Flukes, like other flatworms, have protonephridia, and there is typically a pair of longitudinal collecting ducts. There may be two anterior, dorsolateral nephndiopores (in Monogenea) or a single posterior bladder and nephridiopore (in Trematoda. In the ectoparasites, the protonephridia are probably only osmoregulatory in function. The function of the protonephridia in en-doparasites is still uncertain.
Protonephridia are absent in all nematodes and) patently disappeared with the ancestral mere of the class. Some nematodes have no special excretory system, but many do possess a peculiar system of gland cells, with or without tubules, that has some excretory function. In the class Adenophorea, which includes most marine and freshwater nematodes, there is usually one large gland cell, called a renette gland (Fig. 9-19A), located ventrally in the pseudocoel near the pharynx. The gland cell is provided with a necklike duct that opens ventrally on the midline as an excretory pore.
All members of the class Secernentea, which includes many terrestrial species, have a more specialized tubular system, still composed of only a few cells. Three long canals are arranged to form an H (Fig. 9-19B). Two are lateral and extend inside the lateral longitudinal cords. The two lateral cauls are connected by a single transverse canal, from which a short, common, excretory canal leads to the excretory pore, located ventrally on the midline. In many nematodes, that part of each lateral canal anterior to the transverse canal has disappeared, so the system is shaped like a horseshoe; in others the tubules on one side have been lost, so the system is asymmetrical.
The excretory gland cell or tubules are known to eliminate foreign substances, but may have other functions as well. Ammonia is the principal nitrogenous waste of nematodes and is removed through the body wall and eliminated from the digestive system along with the indigestible residues.
The most common type of excretory organ among coelomate animals is a metanephridium. In contrast to the blind protonephridial tubule, a metanephridial tubule opens internally into the coelom. The opening is often funnel-like and clothed with ciliated perito-num, in which case it is called a nephrostome. In imsegmented coelomates there may be one nephrite or one to several pairs of metanephridia; in segmented groups, such as the annelids, the metanephridia are serially repeated, one pair per segment.
In general, a metanephridium processes coelomic hid. Blood filtrate passes into the coelom at various sites of filtration, depending on the species. For example, in a mollusk part of the heart wall is the major ate of filtration and is composed of podocytes, cells with finger-like processes that interdigitate [Fig. 11-90]. The slits between processes are the sites of titration. Podocytes are found at the filtration sites of many animals, e.g., the glomeruli of the ver-tebrate kidney Coelomic fluid, derived from blood filtrate, passes through the nephrostome into the ciliated nephridial tubule. Here it becomes modified by selective reab-sorption and secretion, and the product is finally expelled through the nephridiopore as urine. The extent of tubular secretion and reabsorption depends in pert on the environment in which the animal lives, i.e., whether it is an osmoconformer or osmoregula-tor. The tubule wall is correspondingly specialized and provided with a vascular backing.
Polychaete excretory organs are either protonephridia or metanephridia (Box 10-2). In primitive polychaetes there is one pair of nephridia per seg-ment, but reduction to few or even one pair for the entire worm has occurred in some families. The anterior end of the nephridial tubule is located in the coelom of the segment immediately anterior to that from which the nephridiopore opens (Fig. 10-2). The tubule penetrates the posterior septum of the segment, extends into the next segment, where it may be coiled, and then opens to the exterior in the region of the neuropodium. Both the preseptal portion of the nephridium and the pos-tseptal tubule are covered by a reflected layer of peritoneum from the septum.
Protonephridia of a type called solenocytes are found in phyllodocids, alciopids, tomopterids, gly-cerids, nephtyids, and a few others. The soleno-cytes are always located at the short preseptal end of the nephridium and are bathed by coelomic fluid. The solenocyte tubules are very slender and delicate and arise from the nephridial wall in bunches (Fig. 10-37) Each tubule contains a single flagellum, and the wall is composed of parallel rods connected by the thin lamellae. The latter represent the fenestrations through which fluid passes; this arrangement is characteristic of other types of protonephridia.
All other polychaetes possess metanephridia, in which the preseptal end of the nephridium possesses an open, ciliated funnel, the nephrostome, instead of solenocytes. Typical metanephridia are found in the nereids, where the nephrostome possesses an outer investment of peritoneum and the interior is heavily ciliated. The postseptal canal, which extends laell next successive segment, becomes greatly coded form a mass of tubules, which are enclosed J thin, saclike covering of peritoneal cells. CoiliJ probably an adaptation that increases the игШ area for tubular secretion or rcabsorption. TkJ phridiopore opens at the base of the neuropcdJ on the ventral side. The entire lining of thetutJ is ciliated.
The metanephridia of most other polycbl differ only in minor details (Fig. 10-38) bill display various degrees of regional restncwl the more specialized families. In the fan worn! where only one pair of functional nephndul main, the two nephridia join at the midline to Л a single median canal, which extends forwrJ open through a single nephridiopore on tbet. Excretory waste is deposited directly ouuide, and fouling of the tube is avoided.
In polychaetes the association of the blood vcs-Klswith the nephridia is variable. The fan worms HKt the arenicolids lack a well-developed nephridial blood supply, and the coelomic fluid must be the principal route for waste removal. In other po-hxhaetes the nephridia are surrounded by a network of vessels. In the nereids the nephridial blood supply is greater in those species that live in brack-lb water.
Many polychaetes, particularly nereids, can tolerate low salinities and have become adapted to life mbrackishsounds and estuaries. The gill (notopo-dullobe) of Nereis succinea contains cells specially fot absorbing ions. A small number of species hve in fresh water. The sabellid Manayunkia spe-:wexample, occurs in enormous numbers in ctrum regions of the Great Lakes, such as around themouth of the Detroit River. There are a few ter-Шіаі polychaetes, all tropical Indo-Pacific nereids, which burrow in soil or live in moist litter.
Chloragogen tissue, coelomocytes, and the intestinal wall may play accessory roles in excretion. Chloragogen tissue is composed of brown or greenish cells located on the wall of the intestine or on various blood vessels. Chloragogen tissue, which has been studied much more extensively in earth-worms (see p. 316), is an important center of intermediary metabolism and hemoglobin synthesis.
The adult oligochaete excretory organs are metanephridia, and typically, there is one pair per segment except at the extreme anterior and posterior ends. In the segment following the nephrostome, the tubule is greatly coiled, and in some species, such as Lumbricus, there are several separate groups of loops or coils. Before the nephridial tubule opens to the outside, it is sometimes dilated to form a bladder. The nephridiopores are usually located on the ventrolateral surfaces of each segment.
In contrast to the majority of oligochaetes, which possess in each segment a single, typical pair of nephridia called holonephridia, many earth-worms of the families Megascolecidae and Glos-soscolecidae are peculiar in possessing additional nephridia, which are multiple or branched. Either typical or modified nephridia may open to the outside through nephridiopores, or they may open into various parts of the digestive tract, in which case they are termed enteronephric. A single worm may possess a number of different types of these nephridia, each being restricted to certain parts of the body.
Earthworms excrete urea, but they are less perfectly ureotelic than are other terrestrial animals. Although urea is present in the urine of Lumbricus and other earthworms and although the level of urea depends on the condition of the worm and the environmental situation, ammonia remains an important excretory product.
Salt and water balance, which is of particular importance in freshwater and terrestrial environments, is regulated in part by the nephridia (Fig. 10-54B). The urine of both terrestrial and freshwater species is hypoosmotic, and considerable reabsorption of salts must take place as fluid passes through the nephridial tubule. Some salts are also actively picked up by the skin.
In the terrestrial earthworms water absorption and loss occur largely through the skin. Under normal conditions of adequate water supply, the ne-phridia excrete a copious hypoosmotic urine. It is not certain whether reabsorption by the ordinary nephridia is of importance in water conservation, but the enteronephric nephridia do appear to represent an adaptation for the retention of water. It passing the urine into the digestive tract, muchl the remaining water can be reabsorbed as it goes through the intestine. Worms with enteronepmi systems can tolerate much drier soils or do J have to burrow so deeply during dry periods.
A few aquatic oligochaete species are capabled encystment during unfavorable environmenrd conditions. The worm secretes a tough, mucca covering that forms the cyst wall. Some specie form summer cysts for protection against desicr*! tion; others form winter cysts when thewatertet perature becomes low.
During dry seasons or during the winter, eaii worms migrate to deeper levels of the soil, dm 3 meters in the case of certain Indian species.Ц moving to deeper levels, an earthworm often» dergoes a period of quiescence and in drypeJ may lose as much as 70 per cent of its water.№ ance is restored and activity resumed as soon» water is again available.
Leech contain 10 to 17 pairs of metanephridia, ill the middle third of the body, one pair segment. As a result of the coelom ton and the loss of septa in the leech body, ihndial tubules arc embedded in connective the nephrostomcs project into the coc-c channels. Each nephrostome opens into a itedcapsule.
In most leeches the cavities of the capsule and idial canal do not connect, and the two of the nephridium may even have lost ictural connection. Many branching, intra-ilar canals drain into the nephridial canal, rpens to the outside through the ventrola-nephndiopore. Secretion into the intracellular iculiisthe initial source of nephridial fluid, Btfleunne is very hypoosmotic to the blood, in-inung reabsorption of salts. The nephridia are important organs of osmoregulation
The function of the nephridial capsules is be-Btdtobe the production of coelomocytes. The coelomocytes are phagocytic and engulf particulate matter, but the eventual fate of the waste-laden cells is not certain. They may migrate to the epidermis or to the epithelium of the digestive tract. Particulate waste is also picked up by botryoidal and vasofibrous tissue of the hirudinid leeches and by pigmented and coelomic epithelial cells of glos-siphoniids and piscicolids.