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excretory system[′ek·skrə‚tȯr·ē ‚sis·təm]
the totality of organs that transport out of the bodies of animals and humans excess water, and products of metabolism, salts, and also poisonous substances introduced into the body or formed in it. In protozoans readily soluble excretions (ammonia, urea) are transported to the external environment by means of diffusion (in marine forms and parasitic forms) or contractile vacuoles, which basically perform the function of osmoregulation (in fresh-water forms). In lower aquatic metazoans (sponges, coelenterates) and in sluggish marine animals (echinoderms), metabolic products, principally ammonia, diffuse through the body sur-face and the walls of cavities connected with the external environment. Specifically, the excretory function in the majority of invertebrates is performed by the intestine.
In the process of evolution a special excretory system became differentiated. In lower worms (Platyhelminthes, Gastrotricha, Kinorhyncha, Rotifera, Acanthocephala, Nemertea) and also in priapulids and some annelids, and in the larvae of Polychaeta and mollusks, the excretory organs are protonephridia, a system of tubules that communicate with the exterior by pores and have blind ends inside the organism, ending in special hollow cells, in which there are bundles of long, sticky cilia (ciliary flame) that continuously execute oscillatory movements. In annelids the blind ends of the protonephridial canals end in groups of bottle-shaped cells (solenocytes), the necks of which each contain a flagellum. In the majority of annelids the excretory system is metanephridia, that is, metamerically arranged paired tubular ectodermal organs that open into coelomic sacs by means of ciliated funnels and open to the exterior sometimes on the surface of the next segment. If mesodermal elements are also part of such excretory organs, this is called nephromixium. Excretory organs that develop from mesoderm—so-called coelomoducts—are present in mollusks: the funnel opens into the pericardial cavity (a vestige of the coelom), and the external opening opens into the mantle cavity. In crustaceans the excretory organs are convoluted tubules that begin with a blind “coelomic” sac and open to the exterior at the base of the antennae or maxillae (hence the term “antennal” or “maxillary” glands). Similar are the excretory organs in Xiphosura (coxal glands), which open to the exterior at the base of the fifth pair of legs. Excretory organs that open to the exterior and through which readily soluble metabolic products are removed, are typical of aquatic invertebrates.
Living on dry land, which requires economy of moisture expenditure, causes a change in the nature of catabolites: readily soluble ammonia and urea are replaced by slightly soluble guanine (in arachnids) or uric acid (in myriapods and insects). Of terrestrial invertebrates, the only ones that have excretory organs of the coelomoduct type are the moisture-loving ones—Onychophora and certain arachnids (for example, Phalangida). Characteristic of terrestrial arthropods is transfer of the excretory function to the walls of the intestine (the walls sometimes become the only excretory organ, for example, in pseudoscorpions) or, more frequently, to the Malpighian tubules—long, blind processes of the intestine on the boundary between the midgut and hindgut, formed as outgrowths of the midgut (in arachnids) or hindgut (in insects and myriapods). The physiological advantage of the Malpighian tubules (when living under conditions of moisture deficiency) lies in the fact that through them readily precipitated metabolic products (such as uric acid) are transported with urinary fluid, not to the exterior but to the hindgut, where absorption of water occurs; the dehydrated excreta, together with undigested food remnants, are carried to the exterior through the anal opening, retention of water in the body of the animal thus being accomplished.
A peculiar type of excretion is the deposit of catabolites (such as uric acid) converted into slightly soluble form in special cells—for example, “collecting kidneys” in nematodes, chloragogen cells in earthworms, interstitial tissue in spiders, fat bodies in insects and myriapods, and hepatic caeca in wood lice—which is also associated with economy of water expenditure during excretion. Also similar to these excretory organs are excretophores, cells that collect excreta and discharge them, for example, into the intestinal lumen (in pseudoscorpions, octocorallia, and others).
Secondary transfer of terrestrial invertebrates to life in the water—for example, insect larvae—led to a reversion of excreta to the type characteristic of aquatic animals (with ammonia as the principal catabolite) and to an increase in the osmoregulatory function of excretory organs (Malpighian tubules and intestine).
Among chordates, in tunicates and ascidians, small collecting sacs serve as excretory organs. (In some tunicates no excretory elements have been discovered.) In the branchial region of the lancelet there are approximately 100 pairs of so-called nephromixia, which, through one opening, open into the peribranchial cavity, and through several others (densely sown with solenocytes) are connected with the body cavity. In vertebrates the organs of excretion are typical coelomoducts, aggregations of which form kidneys. From the kidneys, ureters branch off and open immediately into the cloaca or the urinary bladder, which opens to the exterior through the urethra. In the evolutionary sequence of vertebrates, as well as in the individual development of higher vertebrates, one may observe a succession of three types of kidneys. First to arise are the anterior, or head, kidneys, also called pronephros. Later there develop primary, middle, or trunk kidneys—Wolffian bodies or mesonephros. Lastly there develop secondary, posterior, or pelvic kidneys (metanephros). The coelomoducts of the pronephros and primary kidney, usually a pair in each segment, arise from the so-called nephrotome. Part of the nephrotome, opening into the secondary body cavity, forms a ciliated funnel (nephrostome). Opposite the nephrostomes of the pronephros in the body cavity there usually develops a skein of blood vessels common to all the funnels; through it are filtered fluid excretory products from the blood (aqueous salt solutions), which then enter the funnels. The pronephros, having a segmented structure and extending along the entire trunk, is preserved throughout the whole life of the animal among cyclostomes only in Bdellostoma. In the lamprey the remnants of the pronephros that form the anterior division of the kidney open into the pericardial cavity. In other vertebrates the pronephros serves only as a larval organ. It is especially well developed in the larvae of most fishes and amphibians. The mesonephros functions in all adult forms of fishes and amphibians.Its tubules develop in the embryo similarly to the tubules of the pronephros, differing only in that they open into a finished pronephric duct, which from that time is called the mesonephric duct. In Selachomorpha this duct divides from the anterior end into two ducts: the Wolffian duct, which serves as the ureter of the mesonephros, and the Müllerian duct, which is connected to the remnants of the pronephros and serves as an oviduct in females. In male amphibians the Wolffian duct is connected to the sex gland and simultaneously performs the functions of both ureter and spermatic duct. The presence of Malpighian corpuscles is characteristic of the mesonephros. In Amniota the metanephros replaces the pronephros, which functions during the embryonic period and which in adult females degenerates in any case and in males functions as the sperm-conducting apparatus and is called appendix testis, or epididymis. Embryonically the metanephros forms from a special ureter that grows out of the posterior end of the Wolffian duct. This ureter has several terminal branches at its end, into which renal tubules arising from the nephrogenic tissue open. The convoluted tubules of the metanephros never have funnels and originate as a Bowman’s capsule. In mammals and humans, the capsule is followed by proximal convoluted tubules; these are followed by straight descending and ascending tubules, which form the so-called loop of Henle; finally come distal convoluted tubules and then connecting tubules that empty into the excretory duct. The widened portion of the ureter, from which terminal branches enter the kidney, is called the renal pelvis. The three types of kidney are interconnected by certain transitions. Thus, even in the pronephros there may be a few nodes characteristic of the mesonephros. In the latter the funnels quite frequently disap-pear, and it becomes very similar in structure to the metanephros.
The most probable theory explaining the successive change of kidneys is the theory of replacement of renal tubules. Advocates of this theory believe that ancestors of vertebrates had a long kidney with a pronephros-type structure. In their fishlike descendants, on the dorsal side of the tubules of the pronephros, there developed tubules of an improved type—tubules of the mesonephros, which supplanted the pronephric tubules along the entire trunk, except the anterior portion. In Amniota the tubules of the metanephros, which developed in the posterior section of the mesonephros, gradually supplanted the mesonephros.
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