Gastrointestinal tract

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This feeding strategy depends on surface tension. Physics Formula, Glossary, Exam Papers. The most widely used in the home are salt sodium chloride and sugar, which most homemakers would not consider to be chemical preservatives. Some fishes feed on parasites on the surfaces of other fishes, which benefits all but the parasites. In developing countries, nearly all types of vegetables are eaten soon after they are harvested; unlike cereals, tubers, starchy roots, pulses and nuts, they are rarely stored for long periods with a few exceptions such as pumpkins and other gourds.

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Light intensity The plant can photosynthesize faster as a result of a higher light intensity. As the light intensity decreases the rate of photosynthesis decreases. Light is a limiting factor at low light intensities. There will come a point where any extra light energy will not increase the rate of the reaction. T his is because the enzymes controlling the reaction are working at maximum rate. At this point light is no longer a limiting factor. This is because the plant has to spend a certain amount of time doing nothing, waiting for more carbon dioxide to arrive.

Increasing the concentration of carbon dioxide increases the rate of photosynthesis. Plants need a number of minerals to be healthy. These mineral ions are needed to make certain chemicals or to make certain reactions work properly. Plant absorbs these minerals from the soil when water is absorbed. Transport in plants Transport is the movement or flow of different substances within a living organism. The transport system in plants is the vascular bundles xylem and phloem.

Cambium tissue contains cells which divide by mitosis to produce more phloem and xylem. Xylem Phloem Description Consists of non-living woody lignified cells elements joined together to form continuous tubes vessels Consists of living cells sieve elements Substances carried Sap: Transpiration Water enters the plant via the roots by osmosis. They are then carried up the xylem vessels and lost through transpiration which is when water is lost through the stomata.

When water is lost through the stomata it forces the water to be sucked upwards. Factors affecting rate of transpiration Temperature: The higher the humidity the lower the uptake Air current: Plants with a thick waxy layer will reduce water loss through the leaves. Plants can have needle-like leaves. Hair-like fibre on the leaf traps air close to the leaf. It creates a microclimate around the leaf.

As water is lost from the leaf the microclimate becomes very humid. The hairs prevent this humid air from being blown away. Leaves can be folded. The leaf blade is curled in on itself so that the stomata are on the inside.

This creates a humid micro-climate which slows down water loss. Transport in humans Transportation in humans is done by the circulatory system which involves blood being pumped around the body by the heart. Humans have a double circulatory system which means that the blood is pumped twice around the body - once to the heart and another to the rest of the body. Blood transports O 2 , CO 2 , nutrients, hormones and waste products so the movement must be fast.

The heart is really two pumps stuck together. There are two chambers to each side of the heart. The first chamber is called the atrium and is the smaller of the two chambers.

The larger one is called the ventricle. This chamber is the more powerful of the two as it has to force blood out of the heart. The right side of the heart receives deoxygenated blood from the body and pumps it to the lungs. The left side of the body receives oxygenated blood and pumps it around the body so its force must be stronger. In the heart both sides pump together at the same time.

The blood must flow through the heart in one direction. Blood enters the atria from the veins and is then forced into the ventricles. The ventricles force the blood into the arteries. There are a number of sphincter muscles and valves that prevent blood flowing in the wrong direction. The valves are a little like parachutes. When blood flows the wrong way the valves bulge out, blocking the path.

Heartbeat Involves three distinct stages: The atria and the ventricles relax. The semi-lunar valves close, preventing back flow into the ventricles. The elastic walls of the aorta and pulmonary artery contract, forcing blood towards the body and the lungs. Blood from the veins flows into the atria, which begin to fill. Deoxygenated blood enters the right atrium, and oxygenated blood flows into the left atrium. The atria contract, forcing blood into the ventricles, which then fill up.

Sphincter ring muscles close off the venae cavae and the pulmonary veins prevents backflow of blood from the atria into the main veins. The ventricles contract, forcing blood into the aorta and pulmonary artery.

This happens because the pressure of blood in the ventricles is higher than the pressure in the atria. The valve cord prevents the valve from being pushed back too far. The walls of the aorta and pulmonary artery expand. The heart rate can be measured by the heart pace. There are muscles in the wall of the heart that receive hormones from the brain telling it to speed up or slow down e. The vessel supplying the heart with blood is called the coronary artery. This is one of the most important arteries in the body because it supplies the heart with all the nutrients it needs.

If this artery is blocked the heart will slow down or stop causing a heart attack. This is how coronary heart diseases CHD happen - by the build up of fats inside the vessel. The more the amount of fats build up, the slower the heart pumps and the more easily the heart gets tired.

There are different types of blood vessels. Arteries carry blood away from the heart. These vessels split up into smaller ones called arterioles. Arterioles split up into tiny blood vessels called capillaries. It is from these vessels that movement of particles to and from the blood takes place.

Capillaries join together to form larger vessels called venules which join together to form veins. White blood cells and immunity. Respiration is the chemical breakdown of food molecules to release energy.

Breathing is the mechanical movement to ventilate the respiratory surface. It includes inhaling and exhaling. Gaseous exchange is the diffusion of O 2 on a moist surface into an organism and the diffusion of CO 2 out of the organism. Aerobic respiration Anaerobic respiration. In this process glucose is completely oxidized into carbon dioxide and water. This process is slow and is controlled by many enzymes and the energy produced is not used immediately but stored as ATP.

The energy released from ATP can be used in many activities such as: In this process the energy produced is relatively small and the product is variable. Alcohol can be produced when anaerobic respiration happens during fermentation in yeast. In the human body, lactic acid is a product of anaerobic respiration during heavy exercise.

The lactic acid produced needs to be broken down further by oxygen. That is why we continue to breathe heavily after exercising. The oxygen required for the subsequent breakdown of lactic acid is called oxygen debt. The lungs are located in the chest inside a lubricated membrane called the pleural membrane. This allows the lungs to move freely inside the pleural cavity.

The lungs are connected to the outside via the trachea windpipe. The trachea is a tube kept in a rigid shape due to rings of cartilage. The larynx or voice box is located at the top of the trachea while at the bottom end it branches into two bronchi. These lead into the lungs. The bronchi in turn branch off into smaller and smaller bronchioles. These end in tiny air sacs called alveoli.

It is here that gaseous exchange takes place. The surface area of all these alveoli is very large so as to be able to absorb oxygen very quickly. The lungs are very delicate and can easily be damaged. The cells lining the airways have very tiny hair like structures called cilia on them. These cilia are coated with sticky mucus. The beating cilia force the mucus and any particles of dirt up out of the lungs.

These together increase the volume of the chest. Air is drawn into the lungs because the the pressure inside them is lowered as the chest volume is increased. When we breathe out the diaphragm relaxes as does the intercostal muscles. This decreases the volume of the chest, increasing the pressure. This forces air out of the lungs. So it is the changing volume of the chest which causes air to enter and leave the lungs.

Gaseous Exchange The walls of the alveoli are very thin and so are the walls surrounding the alveoli so that is why diffusion of O 2 and CO 2 can take place. Note that other gases do not diffuse through the walls because the concentration of these gases inside and outside of the body are the same. This is the substance which makes smoking addictive. Nicotine is a stimulant which can make the heart beat faster and increase the amount of adrenaline released. It also makes the smoker more shaky and causes stress.

This is produced due to incomplete burning of the tobacco. This gas binds irreversibly to the haemoglobin in red blood cells preventing them from carrying oxygen. If the smoker is pregnant the baby will get less oxygen than usual.

It prevents the cilia in the lungs from working and so the dirt and tar cannot be removed from the lungs. It also damages the alveoli and decrease the lungs' surface area. Urinary system The urinary system consists of 2 kidneys , 2 ureters, a bladder and a urethra. The job of the kidney is to purify the blood as it enters it. Shortened forms of this life cycle are common, for example some oceanic scyphozoans omit the polyp stage completely, and cubozoan polyps produce only one medusa.

Hydrozoa have a variety of life cycles. Some have no polyp stages and some e. In some species, the medusae remain attached to the polyp and are responsible for sexual reproduction; in extreme cases these reproductive zooids may not look much like medusae. Meanwhile, life cycle reversal, in which polyps are formed directly from medusae without the involvement of sexual reproduction process, was observed in both Hydrozoa Turritopsis dohrnii [25] and Laodicea undulata [26] and Scyphozoa Aurelia sp.

Anthozoa have no medusa stage at all and the polyps are responsible for sexual reproduction. Spawning is generally driven by environmental factors such as changes in the water temperature, and their release is triggered by lighting conditions such as sunrise, sunset or the phase of the moon.

These mass spawnings may produce hybrids , some of which can settle and form polyps, but it is not known how long these can survive. In some species the ova release chemicals that attract sperm of the same species.

The fertilized eggs develop into larvae by dividing until there are enough cells to form a hollow sphere blastula and then a depression forms at one end gastrulation and eventually becomes the digestive cavity.

However, in cnidarians the depression forms at the end further from the yolk at the animal pole , while in bilaterians it forms at the other end vegetal pole. Anthozoan larvae either have large yolks or are capable of feeding on plankton , and some already have endosymbiotic algae that help to feed them. Since the parents are immobile, these feeding capabilities extend the larvae's range and avoid overcrowding of sites.

Scyphozoan and hydrozoan larvae have little yolk and most lack endosymbiotic algae, and therefore have to settle quickly and metamorphose into polyps. Instead, these species rely on their medusae to extend their ranges. All known cnidaria can reproduce asexually by various means, in addition to regenerating after being fragmented. Hydrozoan polyps only bud, while the medusae of some hydrozoans can divide down the middle. Scyphozoan polyps can both bud and split down the middle.

In addition to both of these methods, Anthozoa can split horizontally just above the base. Asexual reproduction makes the daughter cnidarian a clone of the adult. Cnidarians were for a long time grouped with Ctenophores in the phylum Coelenterata , but increasing awareness of their differences caused them to be placed in separate phyla. Modern cnidarians are generally classified into four main classes: Staurozoa have recently been recognised as a class in their own right rather than a sub-group of Scyphozoa, and the parasitic Myxozoa and Polypodiozoa are now recognized as highly derived cnidarians rather than more closely related to the bilaterians.

Stauromedusae, small sessile cnidarians with stalks and no medusa stage, have traditionally been classified as members of the Scyphozoa, but recent research suggests they should be regarded as a separate class, Staurozoa. The Myxozoa , microscopic parasites , were first classified as protozoans. Some researchers classify the extinct conulariids as cnidarians, while others propose that they form a completely separate phylum. Many cnidarians are limited to shallow waters because they depend on endosymbiotic algae for much of their nutrients.

The life cycles of most have polyp stages, which are limited to locations that offer stable substrates. Nevertheless, major cnidarian groups contain species that have escaped these limitations. Hydrozoans have a worldwide range: Stauromedusae , although usually classified as jellyfish, are stalked, sessile animals that live in cool to Arctic waters. Prey of cnidarians ranges from plankton to animals several times larger than themselves.

Coral reefs form some of the world's most productive ecosystems. Common coral reef cnidarians include both Anthozoans hard corals, octocorals, anemones and Hydrozoans fire corals, lace corals. The endosymbiotic algae of many cnidarian species are very effective primary producers , in other words converters of inorganic chemicals into organic ones that other organisms can use, and their coral hosts use these organic chemicals very efficiently.

In addition, reefs provide complex and varied habitats that support a wide range of other organisms. This additional level of variety in the environment is beneficial to many types of coral reef animals, which for example may feed in the sea grass and use the reefs for protection or breeding. Few fossils of cnidarians without mineralized skeletons are known from more recent rocks, except in lagerstätten that preserved soft-bodied animals.

A few mineralized fossils that resemble corals have been found in rocks from the Cambrian period, and corals diversified in the Early Ordovician.

Hydrozoa Hydra , siphonophores , etc. It is difficult to reconstruct the early stages in the evolutionary "family tree" of animals using only morphology their shapes and structures , because the large differences between Porifera sponges , Cnidaria plus Ctenophora comb jellies , Placozoa and Bilateria all the more complex animals make comparisons difficult. Hence reconstructions now rely largely or entirely on molecular phylogenetics , which groups organisms according to similarities and differences in their biochemistry , usually in their DNA or RNA.

It is now generally thought that the Calcarea sponges with calcium carbonate spicules are more closely related to Cnidaria, Ctenophora comb jellies and Bilateria all the more complex animals than they are to the other groups of sponges.

In , it was proposed that Ctenophora and Bilateria were more closely related to each other, since they shared features that Cnidaria lack, for example muscles in the middle layer mesoglea in Ctenophora, mesoderm in Bilateria. However more recent analyses indicate that these similarities are rather vague, and the current view, based on molecular phylogenetics, is that Cnidaria and Bilateria are more closely related to each other than either is to Ctenophora.

This grouping of Cnidaria and Bilateria has been labelled " Planulozoa " because it suggests that the earliest Bilateria were similar to the planula larvae of Cnidaria. Within the Cnidaria, the Anthozoa sea anemones and corals are regarded as the sister-group of the rest, which suggests that the earliest cnidarians were sessile polyps with no medusa stage. However, it is unclear how the other groups acquired the medusa stage, since Hydrozoa form medusae by budding from the side of the polyp while the other Medusozoa do so by splitting them off from the tip of the polyp.

The traditional grouping of Scyphozoa included the Staurozoa , but morphology and molecular phylogenetics indicate that Staurozoa are more closely related to Cubozoa box jellies than to other "Scyphozoa". Similarities in the double body walls of Staurozoa and the extinct Conulariida suggest that they are closely related. The position of Anthozoa nearest the beginning of the cnidarian family tree also implies that Anthozoa are the cnidarians most closely related to Bilateria, and this is supported by the fact that Anthozoa and Bilateria share some genes that determine the main axes of the body.

However, in Katja Seipel and Volker Schmid suggested that cnidarians and ctenophores are simplified descendants of triploblastic animals, since ctenophores and the medusa stage of some cnidarians have striated muscle , which in bilaterians arises from the mesoderm. They did not commit themselves on whether bilaterians evolved from early cnidarians or from the hypothesized triploblastic ancestors of cnidarians.

In molecular phylogenetics analyses from onwards, important groups of developmental genes show the same variety in cnidarians as in chordates. The mitochondrial genome in the medusozoan cnidarians, unlike those in other animals, is linear with fragmented genes. Jellyfish stings killed about 1, people in the 20th century, [62] and cubozoans are particularly dangerous. On the other hand, some large jellyfish are considered a delicacy in East and Southeast Asia. Coral reefs have long been economically important as providers of fishing grounds, protectors of shore buildings against currents and tides, and more recently as centers of tourism.

However, they are vulnerable to over-fishing, mining for construction materials, pollution , and damage caused by tourism. Beaches protected from tides and storms by coral reefs are often the best places for housing in tropical countries. Reefs are an important food source for low-technology fishing, both on the reefs themselves and in the adjacent seas. However, human activities damage reefs in several ways: Some large jellyfish species of the Rhizostomae order are commonly consumed in Japan , Korea and Southeast Asia.

Jellyfish is very low in cholesterol and sugars , but cheap preparation can introduce undesirable amounts of heavy metals. Most stingings by C. A number of Myxozoans are commercially important pathogens in salmonid aquaculture. From Wikipedia, the free encyclopedia. Sponge , Ctenophore , and Bilateria. Journal of Evolutionary Biology. Archived from the original on Retrieved May 15, An introduction to higher-level classification and taxonomic richness" PDF.

Invertebrate Zoology 7 ed. Jellyfish and the origin of triploblasty". The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology. Development Genes and Evolution. Shorter Oxford English Dictionary. Journal of Experimental Biology. Canadian Journal of Zoology. Comparative Biochemistry and Physiology Part A: Sessile animals of the sea shore. Hormones are the chemical integrators of a multicellular existence, coordinating activities from daily maintenance to reproduction and development.

The neurotransmitters released by axons are one class of chemical communicators that act on an adjacent cell, usually a muscle cell or another neuron. Hormones are a mostly distinct class of chemical communicators secreted by nerves, ordinary tissue, or special glands; they act on cells far removed from the site of their release.

They can be proteins, single polypeptides, amines, or steroids or other lipids. Hormones travel to their place of action via the circulatory system and then match their particular configuration with a specific receptor molecule attached to a cell membrane or, more usually, located within the cell. The nervous system coordinates the more rapid activities of animal life, such as movement, while the hormones integrate everything else.

Only the larger, more complex animals, such as vertebrates and some arthropods, have special endocrine glands to produce hormones; other animals use nerve cells or tissues such as the gonads.

Endocrine glands are another example of a partitioning of functions into separate organs, a system that increases efficiency but that requires a relatively large size to maintain. Greater specialization is also associated with greater difficulties in regenerating lost parts or preventing breakdowns in functions. Although the list of hormones found in the mammalian body may seem large, the numbers are surprisingly low for the variety of functions they influence.

Which of the multiple functions any one hormone regulates depends on the specificity of the receptors on or within cells. Because all hormones bathe all cells as a result of their transport by the circulatory system, it is more efficient to have a general messenger transported to a cell, where it elicits only one of many possible outcomes. As in the nervous system, the specificity of response lies in the organ that responds and not with the messenger that merely commands action.

Chemicals that allow communication among individuals are called pheromones. Sexual attractants are the most common, but there are many other kinds. In contrast to plants, the essential nutrients that animals require to sustain life and to reproduce come packaged with their source of energy—the flesh or organic remains of other organisms.

More complex animals tend to shorten and even eliminate many synthetic pathways, because most of the essential building blocks of their own complex molecules are present in their food. Reducing synthetic flexibility, however, inhibits a radical alteration in diet.

The digestive and synthetic chemistry of animals strongly reflects their diets; some of this design may be altered with diet, and some may not. No matter how many leafy vegetables humans consume, for example, the cellulose remains undigested because appropriate microorganisms are not present in the digestive tract and they cannot be obtained at will.

Consequently, essential nutrients are species-specific and tend to include only molecules adequately available in the usual diet. The structure of a digestive system reflects its typical diet. Its purpose is to process food only to the point at which it can be transported to other cells for use as either fuel or structural material. In the simplest animals, such as sponges or some coelenterates, digestion is entirely intracellular, and some of the products of digestion are transported to nondigestive cells.

As animals began to catch larger types of food, more of the digestive process had to be handled extracellularly.

At the simplest level, seen in coelenterates or flatworms, large food items are held in an internal cavity the gut or even externally where certain cells release digestive enzymes. The food is broken down only to the stage at which it can be ingested by cells, which finish the process intracellularly.

In more complex animals extracellular digestion accounts for virtually all breakdown of food before the products are transported to nondigestive cells.

Chemical digestion, whether intracellular or extracellular, is a relatively slow way to decompose a large item. Thus, animals begin to break it apart mechanically before exposing it to digestive enzymes. Teeth, the molluscan radula , and muscular gizzards are organs that speed up the digestive process by macerating food into finer particles. Very early in their evolution animals acquired a one-way gut gastrointestinal system , with the mouth typically armed with the macerating equipment and the terminal stretch sometimes specialized to retrieve excess water or other nutrients.

Often a single passage through the digestive system leaves a great deal of useful material unclaimed. Because food moves along at a characteristic rate, which is sometimes influenced by how much is coming in, not all can be fully digested.

Some animals regularly eat their feces to retrieve nutrients that may have escaped during first passage. If not recycled by their owners, feces are consumed by a diverse set of organisms. A common specialization of the gut is the stomach or crop—a highly extensible part of the digestive tract that is used to hold a large amount of food and partially digest it before it enters the intestines, where most of the chemical breakdown and absorption of nutrients occurs.

Most animals eat intermittently; the less often they eat, the larger the relative stomach size. Internalizing as much food as possible when it is available prevents potential food from being taken by a stronger competitor or enables a feeder to retreat to safety while digesting its meal.

Ceca and second stomachs provide symbiotic microorganisms with a safe area within the gut to digest cellulose. Excess microorganisms mixed in with the partly digestible wastes contribute a steady protein-rich fare to the host in exchange for an optimal place to consume cellulose. Stomachs predominate as a gut specialization because they allow animals to keep food from competitors or other dangers, but a few animals have developed ingenious methods of digesting their food before ingesting it.

Humans are latecomers to this practice and have not yet carried it very far. Starfish exploit secondary radial symmetry and tube feet to open bivalved mollusks only enough to inject their stomachs, digest their meal within the protected shell, absorb the products, and leave the wastes behind. Spiders immobilize prey by silk wrappings and venoms, inject digestive enzymes, and drink the brew. Some primitive animals, like placozoans and certain flatworms, simply hunch over their prey as they digest it externally, a practice that leaves them vulnerable to other predators.

Animals use surfaces in many ways but no more strikingly than in the gut. Nutrients enter the body proper through the surface membrane of the gut; the larger the animal, the larger this surface area must be. The simplest guts, found in animals from sponges to flatworms, simply branch like trees as the animal increases in size; the gut itself reaches all parts of the body to within the distance of a few cells and thus can serve for nutrient transport. As muscle masses become more prominent, the gut is squeezed into a more compact form.

The gut compensates for this lack of space by internalizing its foldings. For example, the lining of the mammalian small intestine , the major site of digestion and absorption, is not only folded but each cell also has numerous outpocketings microvilli , which increase the surface area fold.

Mammals and birds that primarily eat plants have longer intestines than those that favour meat. Warm-blooded animals, which maintain constant internal temperatures, require a great deal more energy than cold-blooded ones and thus tend to concentrate more surface area into a gut.

Although they are not efficient energy users, it is to their advantage to obtain more usable energy even if efficiency is lost in the process. Animals live in an aquatic environment even on land. Each cell is in contact with the ocean or its aqueous equivalent, which carries food and oxygen to the cells of the animal and carries its metabolic wastes away. Smaller animals simply use the fluid-filled coelom for transport.

Increasing size, however, places too many cells beyond diffusion distance from either the coelom or the outside. A muscular pump attached to muscular vessels has arisen in larger animals to move the interstitial fluid surrounding the cells. Most animals have open circulatory systems. Those few animals with closed circulatory systems have a continuous series of vessels to circulate fluid to the vicinity of all cells, whereas those with open systems have vessels only near the heart.

Actually, no system is entirely closed or open. In open systems the interstitial fluid and the circulatory fluid are the same, but in closed systems the two fluids can differ considerably in composition.

Closed circulatory systems have several advantages that make them more appropriate than open systems for large, active animals: For example, cephalopods, alone among the mollusks, and nemerteans, the most active of acoelomates, have closed systems, as do all annelids and vertebrates.

Decapod crustaceans, the largest living arthropods, have nearly closed systems. The most fully open systems have a heart with a few vessels leading from it, while fully closed systems both leak fluid which is reclaimed by the open lymphatic system and have open sections.

For example, blood flow in the vertebrate liver is partly open. In closed systems, blood flow can be both higher and directed more often to tissues that require a greater perfusion of blood.

If blood is confined within discrete vessels, most of which are muscular, contractions can vary the flow rate according to need by altering the amount of constriction. Thus, the heart beats faster during exercise, when the muscles need more oxygen. Fear changes the distribution of blood flow to ready the muscles for possible imminent action. The more muscular arteries, which carry oxygenated blood to the tissues, can proliferate more finely in active tissues so that more cells are closer to the capillaries, where exchange takes place.

Another advantage of a closed system is the ability to carry a high density of oxygen-bearing cells. Such cells cannot flow smoothly through the sometimes tight interstitial spaces and thus are not much used by animals with open systems. A great deal more oxygen, however, can be carried if the oxygen carrier such as hemoglobin is packed into cells. The viscosity of the blood is a function of how many discrete particles are contained within it, and size is of little influence. If all the hemoglobin in the blood of humans were released by dissolving the cell membranes, it would be a thick gel unable to flow.

Animals with open systems do aggregate their oxygen carriers into giant polypeptides, but single molecules have limits to their size. Myriapods and insects, highly active arthropods with open systems, circumvented this problem by evolving a tracheal system of respiration, as have some other groups: A few types of cells protect organisms from a potentially hostile outside environment.

Internal cells thus can eliminate any unnecessary ancestral life-support components as they specialize for various functions. This cooperation maintains an ideal internal environment for the members of the society of cells but only at the cost of active labour and expenditure of energy. Ammonia dissolves readily in water and thus is removed from an animal that needs to rid itself of excess water anyway. In small animals, the ammonia diffuses into the surrounding water. With large size or a need for water conservation, animals excrete urea , a compound less toxic than ammonia but one that also contains carbon and oxygen and thus potential energy.

Urea also is highly soluble in water, but its low toxicity means that it can be concentrated before being excreted. Terrestrial animals with problems of water conservation either convert urea into uric acid , a solid compound that can be stored indefinitely in the body or voided with the feces, or develop efficient excretory organs e.

Although water balance is usually handled by the kidney, salt balance is sometimes a specialized function of other organs. For example, because freshwater fish tend to lose a great deal of salt through their gills, they simply expend energy to concentrate salt against a gradient at this location.

Primitive members of all major taxa of animals reproduced sexually, and virtually all animals still do at some time or another. In contrast to other activities, that of reproduction and life history may be most complex in the more simply structured animals.

Thus, although locomotion constrains the reproductive strategy of an animal, the possibilities with any locomotory mode are diverse. For example, although sessile animals need not expend energy attracting a mate, they do face the problem of getting their gametes in contact with those of the opposite sex.

Sometimes both sexes release gametes in immense swarms in which the probability of contact with the opposite sex is high. Often the female harbours large eggs, and the smaller, more mobile sperm are released to find them. In sponges, sperm simply enter with food. Hermaphroditism the possession of both male and female capabilities and parasitism by males are ways by which sessile, slow-moving, or sparsely distributed animals cope with finding mates.

Barnacles , which are sessile crustaceans, elongate one limb to transfer sperm directly to another barnacle. Some barnacles and other animals have small males that are parasitic on the females. Mobile animals employ many kinds of devices for signaling their availability to the opposite sex. Pheromones, sound, and visual cues are used singly or in combination. Competition for mates may lead to elaborate courtship rituals, which enable a female to choose a suitable male; to size increases of males that fight for control of a harem; or even to size diminution and ultimately parasitism as males compete for a mate.

In some species, sex changes with age, with males turning into females as they get larger. In a few animals, the sex depends on whether the individual settles on the substrate becoming female or on another individual becoming a parasitic male. Finding a mate is but one aspect of a reproductive strategy. The size of eggs is intimately related to the stage of development at which the young emerge to independent life, which in turn correlates with the habitat or mode of locomotion.

For example, marine animals at one extreme produce vast numbers of tiny eggs, which hatch at an early developmental stage e.

Smaller larvae spend more time feeding as plankton before settling down to adult life, and during this time they are vulnerable to predation. However, they can disperse more widely, and their vast numbers give a positive chance that some will survive at each reproductive period. Terrestrial animals always produce relatively large, developmentally advanced young spending the larval time in the egg , because the rigours of living on land demand immediately functional organ systems to sustain a free-living life.

Another problem faced by animals as well as plants is whether to breed only once during life, and thus to put all gathered energy into the effort, or to spend less energy during each reproductive period in order to grow and survive to reproduce for many years. A major factor affecting the evolution of a system of reproduction is whether the adult or the juvenile has the greater likelihood of survival.

Some insects, such as mayflies, spend so little time as an adult not much more than a day that they have lost their feeding structures so as to allot more energy and space to reproduction.

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