Physical Health

Elimination

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How do woodpeckers extract grubs with their tongues? Even if it is highly milled and polished, the parboiled grain still retains the major part of its thiamine and other B vitamins. As with plants, passive settlers do well only with luck. Getting rid of undigested materials. It wasn't clear how they could catch them; birds that hunt flying insects usually have short beaks to help them open their mouths wide.

Types of skeletons and their distribution

Gastrointestinal tract

Rice is a particularly important food for much of the population of China and for many other countries in Asia, where close to half the population of the world lives. It is also important in the diets of some peoples in the Near East, Africa and to a lesser extent the Americas.

Much of the rice is produced in small irrigated fields or paddies in Asia, but some is grown in rain-fed areas without irrigation. The outer layers and the germ together contain nearly 80 percent of the thiamine in the rice grain. The endosperm, though constituting 90 percent of the weight of the grain, contains less than 10 percent of the thiamine. Lysine and threonine are the limiting amino acids in rice.

After harvesting, the rice seeds or grains are subjected to different milling methods. The traditional home method of pounding rice in a wooden mortar and winnowing it in a shallow tray usually results in the loss of about half of the outer layers and germ, leaving a product containing about 0.

The procedure of milling and subsequently polishing rice, which produces the highly esteemed white rice on sale in many shops, removes nearly all the outer layers and germ and leaves a product containing only about 0. This amount is grossly deficient. In Asia, many poor people have a diet consisting mainly of rice for much of the year. A person eating g of highly milled polished rice per day would get only 0. The same quantity of home-pounded or lightly milled rice would provide approximately 1.

Fortification is one method of adding micronutrients. Another way of providing highly milled rice that is reasonably white and yet contains adequate quantities of B vitamins is by parboiling. This process is usually done in the mill, but it can be done in the home. The paddy, or unhusked rice, is usually steamed, so that water is absorbed by the whole grain, including the endosperm. The B vitamins, which are water soluble, become evenly distributed throughout the whole grain Figure The paddy is dried and dehusked, and it is then ready for milling in the ordinary way.

Even if it is highly milled and polished, the parboiled grain still retains the major part of its thiamine and other B vitamins. The solubility of the B vitamins has its disadvantages. Rice that is washed too thoroughly in water loses some of the B vitamins, which are dissolved out. Similarly if rice is boiled in excess water, a considerable proportion of the B vitamins is likely to be discarded with the water after cooking. Rice should therefore be cooked in just the amount of water it will absorb.

If any water is left over it should be used in a soup or stew, since it will contain valuable B vitamins which should not be wasted. Wheat genus Triticum is the most widely cultivated cereal in the world and its products are very important in human nutrition.

In many parts of the world where wheat cannot be grown it is imported and is becoming an increasingly important part of the diet, especially for the urban population. However, importation of wheat, like that of all products, must be offset by adequate exports to prevent a drain on a country's foreign exchange. Bread, usually made from wheat flour, is a popular convenience food.

When purchased, it saves time and fuel for poor families. Pasta is also becoming increasingly popular in some developing countries. Wheat provides a little more protein than does rice or maize, about 11 g per g. The limiting amino acid is lysine. In many industrialized countries wheat flour is fortified with B vitamins and sometimes iron and other nutrients. Effects of milling and parboiling on thiamine in rice. Wheat is usually ground and made into flour.

As with other milled cereals, the nutrient content depends on the degree of milling, i. Low-extraction flours have lost much of their nutrients. In some developing countries where wheat is being increasingly used, the bakers have encouraged the trend towards highly refined products. Traders also prefer the highly milled product because it stores better. Its low fat content reduces the chances of rancidity, and its low vitamin content makes it less attractive to insects and other pests.

Millets and sorghum are cereal grains widely grown in Africa and in some countries in Asia and Latin America. Although less widely grown than maize, rice and wheat, they are important foods. They survive drought conditions better than maize and other cereals, so they are commonly grown in areas where rainfall is low or unpredictable. They are valuable food crops because they nearly all contain a higher percentage of protein than maize and the protein is also of better quality, with a fairly high content of tryptophan.

These cereals are also rich in calcium and iron. Because they tend to be ground at home and not in the mill, they are less frequently subjected to vitamin, mineral and protein losses.

However, in many areas of Africa, they are being replaced by rice and maize, although they usually continue to be grown for making beer. In some parts of Asia millets are regarded as low-class foods for poor people. Many millet and sorghum varieties have the disadvantages of susceptibility to attack by small birds and a tendency to shed their grain. Losses are often high. In some countries millets and sorghum are used mainly to feed animals.

Sorghum Sorghum vulgare or Sorghum bicolor is believed to have originated in Africa but is now cultivated in many countries. It is also called guinea corn or durra, and in India it is known as jowar. There are many varieties of sorghum; most grow tall and have a large inflorescence, but there are also dwarf varieties. The grain is usually large but varies in colour and shape with the type.

Sorghum requires more moisture than millets but less than maize. Sorghum is a nutritious food and many varieties have a higher protein content than other cereals.

There are several species of millet. The most important in Africa are bulrush millet Pennisetum glaucum , also called pearl millet, and finger millet Eleusine coracana. The former, as the name implies, looks rather like a bulrush, but the inflorescence may be much longer and thicker, some times 1 x 8 cm.

The inflorescence of finger millet is shaped like a rather flaccid hand. The seeds are smaller than those of bulrush millet. It is very commonly used for making beer. Oats are not important in the diets of most developing countries. The crop is grown in a few cold highland areas, where it is locally prepared and not usually milled. Oats are a good cereal containing rather more protein than maize, rice or wheat, but they also contain a considerable quantity of phytic acid which may hinder absorption of iron and calcium.

Oatmeal is imported for use in porridge and is used in some manufactured infant foods. Rye is little grown in Africa, Asia and Latin America, and even in Europe it is not an important item of the diet. It has nutritive properties similar to those of other cereals and is sometimes added to bread. Barley is grown in some of the wheat-growing districts of Africa and in highland areas of Asia and South America.

In these places it is usually consumed as a stiff porridge after home preparation. In Europe it is now used mainly for animal feeding and in the preparation of alcoholic beverages such as beer and whisky. This new cereal is a cross between wheat and rye. It has promise of high yields and good nutritive value. It is particularly suited to temperate climates. Teff Eragrostis tef is an important cereal in Ethiopia, where it is held in special regard although it gives a relatively low yield per unit area.

It is usually ground into a flour, cooked and eaten as injera, a type of baked pancake. The nutritive value of teff is similar to that of other cereal grains, except that it is richer in iron and calcium. The high consumption of teff in parts of Ethiopia may be an important reason why iron deficiency anaemia is rarely reported there.

Quinoa is a millet-like cereal grain which is grown in South America in the Andes, particularly in the altiplano. It grows well even where rainfall is low, soil is not very fertile and nights are very cold. As a food it has a special place in the diets of some Andean peoples. A number of edible tubers, roots and corms form an important part of the diet of many peoples in different parts of the world. In tropical countries cassava, sweet potatoes, taro cocoyam , yams and arrowroot are the most important foods in this class.

In the cooler parts of the world the common potato is also widely grown. These food crops are usually relatively easy to cultivate and give high yields per hectare. They contain large quantities of starch and are therefore a fairly easily obtainable source of food energy. As staple foodstuffs, however, they are inferior to cereals because they consist of about two-thirds water and have much less protein, as well as lower contents of minerals and vitamins. They usually contain less than 2 percent protein, whereas cereals contain about 10 percent.

Taro and yams, however, contain up to 6 percent good-quality protein. Although cassava Manihot esculenta , also known as yuca or manioc, originated in South America, it is now widely grown in many parts of Asia and Africa, mainly for its starchy tuberous roots which may grow to an enormous size. Readily established from cuttings, it will grow in poor soil, requires relatively little attention, withstands adverse weather conditions and until recently was not greatly afflicted by pests or disease.

However, in some parts of Africa, notably Malawi, cassava plants in the fields have been attacked and destroyed by mealy bug. Energy yields per hectare from cassava roots are often very high, potentially much higher than from cereals.

The leaves of the plant are eaten by some societies and are nutritious. However, cassava has the great disadvantage of containing little but carbohydrate.

It is especially unsuitable as the main source of energy for the infant or young child because of its low protein content. It should therefore be supplemented liberally with cereals and also with legumes or other protein-rich foods. However, in non-arid areas where the main food and nutrition problems arise from shortage of total food and deficient energy intake, cassava should be encouraged because of its high yields and other agricultural advantages.

Cassava contains less than 1 percent protein, significantly less than the 10 percent in maize and other cereals. It is not surprising, therefore, that kwashiorkor resulting from protein deficiency is much more common in young children weaned on to cassava than in those weaned on to millet or maize.

Cassava also has considerably less iron and B vitamins than the cereal grains. Cassava, particularly bitter varieties, sometimes contains a cyanogenic glucoside. This poisonous substance is present mainly near the outer coat of the tuber, so peeling cassava helps reduce the cyanide. Cassava that is soaked in water or boiled in water that is then discarded also has reduced cyanide levels. In addition, toxicity can be reduced by pounding, grating and fermentation of the cassava roots. Toxic effects tend to occur where these practices are not used.

Cassava consumption has also been linked with goitre and iodine deficiency disorders see Chapter Cassava leaves are frequently used as a green vegetable.

Their nutritive value is similar to that of other dark green leaves. They are an extremely valuable source of carotene vitamin A , vitamin C, iron and calcium. The leaves also contain some protein.

To preserve the maximum quantity of vitamin C in the leaves, they should not be cooked for longer than about 20 minutes. Cassava tubers may be eaten roasted or boiled, but more often they are sun-dried after soaking and then made into a powdery white flour. In some countries cassava is milled commercially. In some of this processing the end product is tapioca, which is mainly cassava starch.

In West Africa cassava is used to make fufu a boiled, mashed product. In some countries, for example Indonesia, cassava is regarded as a poor persons' food, and in others as a famine food. Sweet potatoes originated in the Americas and are now widely grown also in tropical Africa and Asia, usually from stem cuttings. Like cassava, the irregularly shaped, variously sized tubers contain little protein.

They contain some vitamin C, and the coloured varieties, especially the yellow ones, provide useful quantities of carotene provitamin A. Sweet-potato leaves are often eaten and have properties very similar to those of cassava leaves.

However, the leaves should not be picked to excess since, as with other tuber crops, this may reduce the yield of tubers. There are innumerable varieties of yams genus Dioscorea , some of which are indigenous to Africa, Asia and the Americas. They vary in shape, colour and size as well as in cooking quality, leaf structure and palatability. Besides the many domesticated varieties a number of wild varieties are eaten. Yams are more extensively grown in West Africa than in East Africa.

In Nigeria, for example, yams are still an important root crop despite an increase in the popularity of cassava. Yams require a warm, humid climate and soil rich in organic matter; these requirements limit their cultivation.

The proper cultivation of yams entails initial deep digging and subsequent staking of the twining vine-like plant. The work involved is more arduous than for cassava, and the yields, though high, are usually a little lower than cassava yields. Yams usually contain about twice as much protein 2 percent as cassava, although very much less than cereals.

It is widely grown and consumed in the Pacific islands. In Africa, taro is common in forest areas e. Taro is often grown in association with bananas or plantains e. The plant has large "elephant ear" leaves.

Both the tubers and the leaves are eaten. The nutritive value of taro is similar to that of cassava. In some areas taro is being replaced by tania or new cocoyam Xanthosoma sp. Potatoes were first taken to Europe from South America and became a cheap, useful, high-yielding alternative to the existing main staples, just as cassava replaced millet in parts of Africa and Asia.

However, the mistake of relying almost entirely on one crop was emphasized by the great Irish potato famine of the nineteenth century: Potatoes remain a very important food of people living in the Andean countries of South America. Much research on this crop has been conducted in Peru.

From Europe, potatoes travelled to Africa and Asia, where they are grown in higher cool areas. If well cultivated in the right soil and climate, they can give a very high yield per hectare.

Like other starchy tubers, potatoes contain only about 2 percent protein, but the protein is of reasonably good quality. Potatoes also provide small quantities of B vitamins and minerals. They contain about 15 mg vitamin C per g, but this amount is reduced in storage. The keeping quality of potatoes is not good, unless they are stored carefully. Arrowroot, which is grown in areas with adequate rainfall, is liked by certain peoples in Africa and Oceania. The nutritive value of arrowroot is similar to that of potatoes.

The roots are eaten in a variety of ways, often roasted or boiled. Strictly speaking, bananas and plantains should be discussed with fruits; from the nutritional point of view, however, they are more appropriately considered under starchy foods. It is difficult to differentiate among the many varieties of plantain and of banana.

For the purposes of this book, plantains may be described as bananas that are picked green and are cooked before eating. Plantains contain more starch and less sugar than bananas, which are usually eaten raw like other fruits. Bananas and plantains originally grew wild in damp, warm forest areas. They have probably been used as food by humans since earliest times. Bananas and plantains are now cultivated extensively in many of the humid tropical areas.

Some peoples such as the Buganda in Uganda and the Wachagga in the United Republic of Tanzania depend on plantains as their main food. A g portion of green bananas or plantains provides 32 g carbohydrate mainly as starch , 1. Plantains also have a high water content. Their very low protein content explains why kwashiorkor commonly occurs in young children weaned on to a mainly plantain diet.

Bananas usually contain about 20 mg vitamin C and mg vitamin A as beta-carotene equivalent per g. For this reason fresh fruits and vegetables are much less important in the diet for those whose staple food is banana than for those whose staple is a cereal or root.

Bananas are, however, low in their content of calcium, iron and B vitamins. As bananas supply only 80 kcal per g, about 2 kg must be eaten to provide 1 kcal. Plantains are usually picked while they are still green. The skin is peeled off and they are then either roasted and eaten, or, more commonly, cut up, boiled and eaten with meat, beans or other foods. Plantains are frequently sun-dried and made into a flour.

The trees are widely grown in Indonesia, but sago as a food is particularly popular in certain Pacific islands. Sago has low protein content. Sugar, as sold in shops, is almost percent sucrose and is essentially pure carbohydrate. In Africa, Asia and Latin America nearly all locally produced sugar comes from sugar cane, while in Europe and North America some comes from sugar beet. In areas where much sugar cane is grown, the consumption of sugar or sugar-cane juice chewed cane is often high.

In other parts of the world the consumption of sugar tends to rise with economic advancement. In the United States and the United Kingdom in about 18 percent of energy consumed came from sugar sucrose , mainly in sweetened foods. In contrast, in many African countries less than 5 percent of energy comes from sucrose.

Sugar is a good and often inexpensive source of energy and can be a valuable addition to bulky energy-deficient diets. Contrary to popular belief, customary consumption of sugar is not related to obesity, diabetes, hypertension or any other non-communicable disease.

Frequent sugar consumption can be associated with dental caries when coupled with poor oral hygiene, but sucrose is no more cariogenic than other fermentable sugars. White sugar contains no vitamins, protein, fat or minerals. Many people find that its sweet taste adds to the enjoyment of eating.

The yields of energy per hectare of land are very high on productive sugar estates. From time immemorial honey has been extensively gathered in developing countries from wild hives.

Now more and more hives are being kept, often in hollowed and suspended pieces of tree-trunk or in other more managed ways. The incentive to keep bees tends to be the high price of beeswax rather than just the honey. Honey has gained the false reputation of being of special nutritive value. In fact it contains only sugar carbohydrate , water and minute traces of other nutrients. Although merely a source of energy, it has sensory value as a pleasant food for humans. Beans, peas, lentils, groundnuts and their like belong to the botanical family of Leguminosae.

Their edible seeds are called legumes or pulses. Agriculturally the plants of this group have the advantage of being able to obtain nitrogen from the air and also add some to the soil, whereas most other plants take nitrogen from the soil and do not replace it.

Legumes usually thrive best when they can get water early in their growth and then have a warm dry spell for ripening. They are therefore often planted at the end of the rains to ripen early in the dry season. In Africa, Asia and Latin America the seeds are usually left on the plant to reach full maturity and are then harvested and dried.

Some may be picked earlier and eaten while partly green, as in Europe and North America. Some varieties are susceptible to attack by weevils; spending a small amount of money on insecticides to prevent this is definitely economically sound. However, care must be taken to ensure that an excess of insecticide is not applied, that the insecticide is relatively safe and that the beans are well washed before cooking.

The legumes are very important from a nutritional point of view because they are a widely available vegetable food containing good quantities of protein and B vitamins in addition to carbohydrate. Some legumes, such as groundnuts and soybeans, are also rich in oil.

They usually supplement very well the predominantly carbohydrate diet based on cereals. Most legumes contain more protein than meat, but the protein is of slightly lower quality because it has less methionine.

However, when pulses and cereals are eaten together at one meal they supply a protein mixture containing good quantities of all the amino acids, which improves the protein value of the diet.

Legumes also contain some carotene provitamin A and ascorbic acid if eaten green. Similarly, dried legumes allowed to sprout before eating have good quantities of ascorbic acid. Some legumes contain antivitamins or toxins see Chapter Unless there is a very good reason for introducing a new crop such as soybeans, it is more sensible to encourage increased production and consumption of whatever legume is already grown and popular in any area.

The local people will have a taste for it, and agricultural conditions are usually suitable. It is also highly important to try to introduce beans and other pulses into the diet of children at an early age. Children are just as able as adults to digest beans easily. A wide variety of beans, peas, lentils, grams, etc.

All three regions have indigenous legume varieties but also grow varieties that originated on other continents. There are many kinds of beans. Haricot or kidney beans Phaseolus vulgaris were originally from the Americas but are now widely grown in Asia and Africa. Broad beans Vicia faba are more common in temperate areas. Lima beans Phaseolus lunatus originated from Peru but are eaten all over the tropics and subtropics. Mung beans Phaseolus aureus , indigenous to the Indian subcontinent, are small seeds but very popular.

Scarlet runner beans Phaseolus multiflorus are popular as a fresh vegetable in Europe and North America, but the large mature seeds are eaten dried in many countries. Lentils Lens esculenta and some similar legumes often known as grams are very important in the diets of people in many developing countries.

Lentils have been cultivated for food by humans for thousands of years. The plants are of small size, as are the seeds. Grams include the important pigeon pea Cajanus cajan , chickpea Cicer arietinum and green gram or mung bean Phaseolus aureus. In many South Asian countries various dhals made from these legumes form a significant part of the diet, providing important nutrients to supplement the staple food, which may be rice or wheat.

In many parts of Africa both cowpeas and pigeon peas are grown and consumed. The pigeon pea is perennial and relatively drought resistant. Lathyrus sativus, another drought-resistant legume, is grown widely in India, but consumption of large amounts can lead to the severe toxic condition called lathyrism see Chapter Winged bean Psophocarpus tetragonolobus is another important legume with a very high protein content 35 percent , but it is not yet widely grown.

Peas are commonly consumed as a green vegetable fresh, canned or frozen in Europe and North America and by more affluent people elsewhere.

In developing countries the seeds are allowed to mature and are dried and consumed in the same manner as other legumes. These legumes excluding soybean all have a somewhat similar nutritive value, but the mature beans are eaten in a variety of ways and have different flavours and other culinary qualities.

Most legume seeds usually contain about 22 percent protein as opposed to 1 percent in cassava roots and 10 percent in maize and good quantities of thiamine, riboflavin and niacin; in addition they are richer in iron and calcium than most of the cereals.

The large number of other legume seeds of various shapes, colours and sizes on sale at food shops or marketplaces in almost any village or town in tropical countries is evidence of an appreciation of dietary variety and culinary finesse. Culture and local taste importantly determine how these foods are eaten. However, the soybeans produced by these countries are mainly used commercially for oil and as animal feeds. Asia still produces much of the soybeans for direct human consumption.

They are not widely grown in Africa or Latin America. Soybeans contain up to 40 percent protein, 18 percent fat and 20 percent carbohydrate. The protein is of a higher biological quality than that from other plant sources. Soybeans, used in a wide variety of ways, are very important in the diets of the Chinese and in those of some other Asian countries.

In China soybeans are made into a variety of tasty dishes which supplement the staple food of rice or other cereal. Soy products such as tofu soybean curd and tempeh a fermented product are important in Indonesian cuisine and popular elsewhere. Soybeans have not become a popular food in Africa or Latin America, where there is little local knowledge of the best methods of preparing them.

People lacking experience with soybeans find them difficult to prepare and cook. Where soybeans are grown they can be locally processed for use in the country as an enrichment of cereal flours, as an infant food or for institutional and school-feeding purposes.

The oil can be exported and the protein-rich residue cake can be utilized in the country. The term "groundnut" is a misnomer since, although botanically a nut, the groundnut Arachis hypogaea is a true pulse, a member of the Leguminosae family. It originated in Brazil but is now extensively grown in warm climates around the world.

It is an unusual plant in that the flower stalk bearing the ovary burrows into the ground, where a nut containing the seed or seeds develops. Groundnuts contain much more fat than other legumes, often 45 percent, and also much more niacin 18 mg per g and thiamine, but relatively little carbohydrate 12 percent.

The protein content is a little higher than that of most other pulses 27 percent. Groundnuts are an unusually nutritious food with more protein than animal meat. They are energy dense because of their oil, and they are rich in vitamins and minerals. As suggested in Chapter 9, if every child, woman and man in Africa ate a handful of groundnuts per day in addition to their normal diet, Africa would be rid of most existing malnutrition.

Groundnuts are fairly widely grown in the tropics. Farmers should produce them for home consumption as well as for cash crops, since they form a very useful addition to the primarily cereal or root diets of many poor families.

They supply much-needed fat, which is high in energy and assists in the absorption of carotene as well as serving other functions. In predominantly maize diets, relatively small quantities of groundnuts, with their high content of niacin and also of protein including the amino acid tryptophan , can prevent pellagra.

When groundnuts are added to children's diets, their high protein and energy content serves to prevent protein-energy malnutrition. However, groundnuts are often grown mainly as a cash crop even in developing countries.

The world's largest producer is the United States. Groundnuts are usually utilized for oil extraction, and the residue, groundnut cake, is used for animal feed.

In the United States a good proportion is consumed as peanut butter. In many countries groundnuts are consumed roasted, boiled or cooked in other ways. Groundnuts, if damaged during harvesting or if poorly stored in damp conditions, may be attacked by the mould Aspergillus flavus. This fungus produces a poisonous toxin known as aflatoxin, which has been shown to cause liver damage in animals and to kill poultry fed on infected groundnuts.

It may be toxic also for humans and may be a cause of liver cancer see Chapter The bambara groundnut Voandzeia subterranea originated in Africa and is grown widely. It resembles the groundnut physically but is not nutritionally similar, having only 6 percent fat.

Its protein content of 18 percent is a little lower than that of most other pulses, but it has about the same mineral and vitamin content as beans. Because of the lower fat content the crop is not in great demand for oil production. Therefore, instead of being sold as a cash crop, it is more often used locally for food.

Coconut is the most important nut crop in Africa. Its origins are uncertain. The nut, being light and impervious to water, no doubt drifted across many seas to germinate on a new shore. It is now extensively cultivated. The tree that bears it is a picturesque and highly useful plant, apart from the food it provides for humans. When it is green, the nut contains about half a litre of water; this is a very refreshing and hygienic drink, but apart from a little calcium and carbohydrate, it has no nutritive value.

The white flesh, however, is rich in fat. The flesh of the coconut is usually sun-dried into copra. The oil from copra is used both for cooking and for making soap. Copra itself is used in the tropics and elsewhere as an addition to many dishes. It is an important ingredient in a variety of cuisines from Thailand to Saudi Arabia. Coconut oil has the disadvantage of containing a relatively high proportion of saturated fatty acids.

The coconut sap in many countries is fermented to yield alcoholic beverages. The cashew nut is produced on a small tree that originated in dry areas of the Americas. Next page Types of waste: Learn More in these related Britannica articles: The biosphere is a global ecosystem composed of living organisms biota and the abiotic nonliving factors from which they derive energy and nutrients. The blood carries the waste products of cellular metabolism to the excretory organs.

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The main route of drug excretion is through the kidneys; however, volatile and gaseous agents are excreted by the lungs. Small quantities of drugs may pass into sweat, saliva, and breast milk, the latter being potentially important in breast-feeding mothers. Some chemicals, rapidly excreted in the urine, must act quickly while they remain transiently in the body.

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Contact our editors with your feedback. Introduction Elimination Biological significance of elimination Types of waste: You may find it helpful to search within the site to see how similar or related subjects are covered. Any text you add should be original, not copied from other sources. At the bottom of the article, feel free to list any sources that support your changes, so that we can fully understand their context.

Internet URLs are the best. Thank You for Your Contribution! The organ systems typical of all but the simplest of animals range from those highly specialized for one function to those participating in many. The more basic functional systems are treated below from a broadly comparative basis.

A skeleton can support an animal, act as an antagonist to muscle contraction, or, most commonly, do both. Because muscles can only contract, they require some other structure to stretch them to their noncontracted relaxed state. Another set of muscles or the skeleton itself can act as an antagonist to muscle contraction. Only elastic skeletons can act without an antagonist; all antagonistic muscles act through a skeleton, which can be either rigid, flexible, or hydrostatic.

Hydrostatic skeletons are the most prevalent skeletal system used by animals for movement and support. A minimal hydroskeleton resembles a closed container. The walls are two layers of muscles antagonists oriented at right angles to one another; the inside contains an incompressible fluid or gel.

The contraction of one set of muscles exerts a pressure on the fluid, which is forced to move at right angles to the squeezing antagonist. The movement of the fluid stretches the other set of muscles, which can then contract to stretch its antagonist back to its relaxed position.

The net result is an alternating change in the shape of the container. Locomotion as varied as crawling, burrowing , somersaulting, looping, or even walking is possible when the container has some means of traction against a substrate: Hydroskeletons are also important in nonlocomotory muscular systems, such as hearts or intestines, which move blood or food, respectively.

Contraction-relaxation cycles push in one direction only when the system has structures that prevent backflow. Hydroskeletons become less efficient when fluid is lost. The optimal volume of fluid for a particular system must remain constant for effective contraction and expansion of the antagonistic muscles. If too much fluid is lost, the animal becomes limp and neither muscle can stretch; when too much fluid is gained, the animal becomes bloated and neither muscle can contract.

Those coelenterates that use a hydroskeleton regularly face a loss of pressure because their skeleton is also their gut. Freshwater animals tend to become bloated as water diffuses into their salty cells, but terrestrial animals with hydroskeletons tend to become limp as they dry.

Solutions to water loss tend to be partial because impermeable barriers, such as a shell, tend not to be very flexible, thus negating the use of a hydroskeleton for movement. Terrestrial animals with locomotory hydroskeletons e. Partitioning a hydroskeleton into many small, separate, but coordinated units facilitates locomotion. In an earthworm , for example, a front group of segments narrows together, thereby elongating that part of the worm.

If there were no partitions between the segments, the fluid would flow farther back, providing little elongation. Widened segments behind these initial segments anchor the worm, and its head moves forward. The process then reverses in a wave, and the posterior end moves forward. Metamerism , or the partitioning of the coelom, is thought to have evolved in ancestral annelids to improve their ability as burrowers in the bottom mud of the ocean. It undoubtedly explains the unrivaled success of this phylum among worms and helps to explain the extraordinary success of one of its relatives, the arthropods, which remained segmented even after the skeletal function of the coelom was lost.

Elastic skeletons do not change shape but simply bend when a muscle contracts. Muscle relaxation results either from a muscle contracting in the opposite direction to its antagonist or from the skeleton resuming its original position.

The tentacles of many hydrozoan coelenterates, the mesoglea of jellyfish , the hinge of clamshells, and the notochord of chordates are examples. The high-pressured coelom contained in the rigid but flexible cuticle of nematodes also functions like an elastic skeleton.

Rigid, jointed skeletons achieve movement through a lever system. The elements of the skeleton are rigid segments attached together by flexible joints.

Muscles span the joints and attach at each end to different elements. The more stable attachment site of a muscle is called the origin, the other the insertion. One muscle contracts and moves the skeletal element on which it is inserted, and an antagonistic muscle contracts and moves the skeletal element in the opposite direction. The biceps and triceps of the upper arm in humans are such a set of antagonistic muscles that bend and straighten, respectively, the lower arm.

The control of movement can be quite precise with jointed skeletons. Muscles can bend or rotate skeletal elements whose length, shape, and number contribute to the resulting action.

The dexterity of the hands is an example of the complexity of controlled movements made possible by a jointed skeleton. Important to the speed and force of a movement are the length of the skeletal element and the size of the contracting muscle.

Short limbs with thick muscles have more power than long limbs with slender muscles, but the latter have more speed. Limbs thus reveal a great deal about how an animal moves. Likewise, the relative massiveness of jaws reflects the toughness of the food eaten. Two animal phyla, Chordata vertebrates only and Arthropoda , exploit jointed skeletons.

Although the skeleton is internal in vertebrates and external in arthropods, the principles of movement are the same. A jointed skeleton is ideal for moving on land because adaptations for protection against dehydration such as the cuticle do not interfere with the action of the skeletal system.

Indeed, the arthropod cuticle serves jointly a protective and a skeletal role. Moreover, the diverse range of precise movements made possible by this skeleton facilitates all sorts of locomotory patterns: Jointed skeletons are also used directly for feeding jaws. Arthropod jaws are derived from legs, while vertebrate jaws are derived from gill arches.

Although all animals can move, not all locomote or displace the body over a distance. Locomotion serves the animal in finding food and mates and in escaping predators or unsuitable habitats. These functions of locomotion are typically correlated among different animals, so that those using the same mechanism of locomotion usually also feed, seek mates, and avoid danger in similar ways. Some of the correlations between mode of locomotion and mode of feeding are described here, but space precludes discussion of the rich diversity found among animals past and present.

Locomotory strategies for finding or gathering food include the following techniques. Sitting still and waiting for food to arrive is particularly prevalent in aquatic habitats but is not rare on land. Sessile animals tend to develop strong defenses that are sometimes incompatible with effective locomotion.

They rely on water or air currents or on the locomotion of their potential prey to bring food within reach. Because food may come from any direction, many sessile animals evolve radial symmetry. Settlement may be permanent or temporary, but in all cases one stage of the life cycle is capable of moving actively or passively from its place of origin.

The choice of attachment site can also be active or passive; passive choice is often associated with an ability to grow in such a way as to maximize feeding efficiency.

As with plants, passive settlers do well only with luck. The retention of locomotory capabilities requires energy and nutrients that can otherwise be diverted into growth or the production of offspring.

Sessile feeders need to move if feeding and resting sites differ. Sessile animals include filter feeders, predators, and even photosynthesizers; the latter include corals that house symbiotic algae.

Internal parasites are usually sessile because they live within their lifetime food supply. Mobile animals that pursue sedentary strategies for seeking prey include web-spinning spiders a terrestrial mode of filter feeding or deep-sea fishes with morphological adaptations that lure prey.

Burrowing animals typically eat the rich organic substrates they move through. Others burrow for protection and either temporarily emerge and gather organic sediments at the top of their burrows or pump water with potential food through the burrow. Instead of digging or finding burrows, some animals move into the centre of sponges, where they find protection and a renewing source of food.

Active movement in search of food requires energy, but this expenditure is more than made up for by an ability to seek out areas of concentrated food. This method of feeding applies to burrowing animals that eat the substrate through which they move, as well as to animals that move over solid surfaces, swim, or fly. Actively moving animals can feed on organisms that do not move, a rich variety coating virtually the entire solid surface of Earth , from the depths of the oceans to the peaks of many mountains.

The main problem with this most productive avenue of food gathering is protection. Shells and poisons are the major types of defenses, although innovative detoxification metabolism and jaws of various kinds breach the defenses in part.

This is an escalating battle in which the defenses, as well as the weapons to penetrate them are continually improving. Nudibranchs, shell-less marine snails, incorporate the defensive stinging cells of prey cnidarians into their own skin. Poisonous plants are eaten by specialized insects that avoid or detoxify the poison.

In fresh water, for reasons not known, the arms race has not proceeded as far as in the sea. Cooperation of individuals enables social animals to obtain food in novel ways. Uncannily like humans, some ants farm and herd other organisms for food. For example, some cultivate a fungus on leaves they cannot directly digest, while others herd aphids from which they milk nectar actually the phloem sap of plants. Some ants even raid the nests of other species and make slaves of them.

Another form of cooperation is the mutualism between species that trade advantage for advantage. Some fishes feed on parasites on the surfaces of other fishes, which benefits all but the parasites. In many animals, including termites and ruminants, microorganisms thrive in the gut and digest cellulose for them.

Coherent movement results only when the muscles receive a sensible pattern of activating signals for example, antagonists must not be activated to contract simultaneously. Animals use specialized cells called neurons to coordinate their muscular activity; nerves are bundles of neurons or parts thereof.

Neurons communicate between cells by chemical messengers, but within a single cell often extremely long they can send high-speed signals through a wave of ionic polarization analogous to an electric current along their membranes, a property inherent in all cells but developed for speed in nerve cells by special modifications.

A system of communication requires three parts: In animals, sensory nerves and organs such as eyes collect the information; associative nerves usually concentrated into a brain integrate , evaluate, and decide its relevance; and effector or motor nerves convey decisions to the muscles or elsewhere. Although all three parts of the nervous system have kept pace with increases in the size and complexity of animals, the simplest systems found among animals those of parazoans and coelenterates are nevertheless capable of intricate feats of coordination.

All ends of a coelenterate bipolar neuron can both receive and transmit an impulse, whereas the unipolar neurons of more derived animals receive only at one end dendrite and transmit at the other axon. A neuron can have multiple dendrites and axons. The earliest animals were probably radial in design, so that bipolar neurons arranged in a netlike pattern made sense.

In such a design, a stimulus impinging at any point on the body can travel everywhere to alert a simple array of myofilaments to contract simultaneously. In the case of directed locomotion and relevant sensory input received at the head end of a bilateral animal, unidirectional transmission of nerve impulses to muscles becomes the only way to communicate effectively.

The location of the brain in the head also reflects efficiency and the speed of receipt of information, because this position minimizes the distance between sensory and associative neurons as well as concentrates these two functions in a small, protected part of the body.

In most animals nerve cells cannot be replaced if lost, although axons can be. Nerve cells tend to be concentrated centrally in ganglia or nerve cords, with long axons extending peripherally.

Although certain animals may lose tails or limbs to predators or in accidents and then regenerate them, loss or damage to the central nervous system means death or paralysis.

The nervous system uses the transmission properties of neurons to communicate. To pass to the next cell at a synapse, where an axon meets a dendrite, a chemical transmitter is required. Although chemical transmission is considerably slower than the ionic wave, it is more flexible.

For example, learning involves in part increasing the sensitivity of a particular nerve pathway to a stimulus. The sensitivity of a synapse can be altered by increasing the amount of transmitter released from the axon per impulse received, increasing the number of receptors in the dendrite, or changing the sensitivity of the receptors.

Bridging the synapse directly by the formation of membrane-bound gap junctions , which connect adjacent cells, enables an impulse to pass unimpeded to a connecting cell. The increase in speed of transmission provided by a gap junction, however, is offset by a loss in flexibility; gap junctions essentially create a single neuron from several.

The same result can be achieved more effectively by lengthening the axons or dendrites, making some nerve cells metres in length. Situations arise where gap junctions become desirable, however. Gap junctions are found in vertebrate cardiac and smooth muscles, both of which transmit impulses along their cells to others.

This ability makes these muscles somewhat independent of nervous-system control.

Biological significance of elimination