Comparative Anatomy.

Comparative Anatomy. I INTRODUCTION Comparative Anatomy, scientific study of the similarities and differences in the structure of living things. Comparative anatomy helps to show how organisms function, how they develop, and how they are linked by evolution, the process by which organisms change over many generations. The theory of evolution, one of the fundamental tenets of modern biology, states that new types of organisms develop from common ancestral types over long periods. Studying the body structures of various organisms often helps scientists determine how different species, or distinct kinds of organisms, are related to each other, as well as how and when they diverged from a common ancestor (see Species and Speciation). Comparative anatomy can be used to investigate plants and simple microorganisms, but its most important role is in the study of animals. In animals, comparative anatomy usually focuses on living species, but scientists also investigate extinct species by examining fossils, body remains trapped in sediment or amber. With extinct animals, anatomists rarely have a chance to study soft body parts because these parts normally decay before they have a chance to fossilize. With living species, the entire body can be examined, giving a much fuller picture of how it functions. Anatomists also compare existing species with fossils to trace the path that evolution has followed and to gather information that is used in animal classification. Anatomical studies usually involve adult animals, but anatomists also investigate the way animal bodies reach their adult shape in a field of study called developmental anatomy. Many important physical features can be seen on the outside of animal bodies, but often the most revealing ones are hidden inside. These hidden features provide valuable clues about an animal's distant ancestors. For example, an endangered reptile from New Zealand called the tuatara looks much like a lizard, and it was originally classified as a lizard back in the early 19th century. But in 1867 anatomist Albert Gunther, working at the British Museum in London, noticed that tuataras have some unusual features. Among these features are teeth that are permanently fused to their jaws rather than separate from the jaw like the teeth of lizards. From this evidence and other anatomical observations he concluded that tuataras are not lizards at all, but sphenodonts--the only surviving members of an ancient group of reptiles that flourished alongside the dinosaurs. Most comparative anatomy studies involve gross anatomy, which deals with structures that are big enough to be seen with the naked eye. Smaller structures, such as individual cells, may also be investigated using the magnifying power of various types of microscopes. This field of study is called microscopic anatomy. In recent years, progress in molecular biology has enabled scientists to investigate still smaller structures, particularly deoxyribonucleic acid (DNA), the hereditary material in all living cells. DNA is made up of strings of four different subunits called nucleotide bases. Anatomists sometimes study the arrangement or sequence of these nucleotide bases in the DNA from different animals, looking for similarities and differences that provide clues to evolutionary family trees. II THE ANIMAL KINGDOM Comparative anatomy is used in the study of all animal groups, but more work has been carried out on some animals than on others. Among invertebrates, or animals that lack a backbone, anatomists focus on a few major groups. Arthropods, which include crustaceans and insects, draw the attention of anatomists who are interested in finding out how the same basic body plan of a segmented body and jointed legs could give rise to such a stunning array of variations. The mollusks, a group of invertebrates that includes snails, clams, squids, and octopuses, have also been thoroughly studied. Squids and octopuses are of particular interest to scientists because they have the most highly developed nervous systems of all invertebrates and large eyes that work very much like those of humans. The anatomy of flatworms and roundworms has been thoroughly investigated because these two groups include many parasitic species, including some that infect humans (see Parasite). Although invertebrates make up over 95 percent of animal species on Earth, work on their anatomy is still dwarfed by the studies carried out on vertebrates, animals that have internal bony skeletons. This is partly because these bony skeletons have left a fossil record of unparalleled richness, which anatomists draw on when comparing one species with another. In addition, vertebrates are the group to which humans belong, so anatomists are interested in studying them to find out how humans evolved. Anatomical studies of vertebrates show how the same underlying body systems have adapted to life in water, on land, and in the air. In laboratory studies, a handful of animals--including the dogfish (a type of small shark), frog, pigeon, and rat--are used as standard examples of vertebrate anatomy. Anatomists also have studied thousands of other species in detail in an effort to piece together exactly how vertebrates have evolved. Some of that history has been pieced together by studying tunicates and lancelets, sea-dwelling invertebrates that are closely related to vertebrates. Despite being brainless and boneless, tunicates and lancelets clearly show some of the physical characteristics that were key to vertebrate success. For example, tunicates use a series of slits for feeding. These slits developed into gills in fish, resulting in an efficient mechanism for extracting oxygen from the water. Lancelets have a stiff structure called a notochord, which enables them to swim efficiently. The vertebral column, which has replaced the notochord in vertebrates, is even more efficient. Such characteristics allowed vertebrates to diversify rapidly and become the most complex animals on Earth. III PRINCIPLES OF COMPARATIVE ANATOMY Despite the variety and complexity of animal life, several key anatomical features divide up the animal world. One of these features is symmetry, meaning that an animal's body parts are the same in size, shape, and position on either side of a dividing line or central axis. Several groups of marine animals--including the cnidarians (jellyfish, sea anemones, and corals), comb jellies, and echinoderms (sea stars, sea urchins, and their relatives)--are radially symmetrical. Their body parts are arranged around a central axis like spokes in a wheel. Almost all other animals, including vertebrates, are bilaterally symmetrical, with two halves arranged on either side of a central dividing line. Bilateral symmetry is often not quite as perfect as it seems. The human body looks more or less symmetrical from outside, but many internal organs are arranged in an asymmetrical way. For example, the liver lies mostly on the right side of the body's dividing line, while the stomach is mostly on the left. In some animals, asymmetry goes much further. A sperm whale has a single blowhole on the left side of its head, while a fish known as a winter flounder has both eyes on the right side. Male fiddler crabs have one small pincer, which is used for feeding, and one giant one, which is used for signaling during courtship. This giant pincer can be either on the right or the left, and it often weighs as much as the rest of the body put together. Some bilateral animals, notably annelid worms (such as earthworms) and arthropods, show a characteristic known as segmentation. Segments, known to biologists as metameres, repeat from front to back of the animal's body. The segments are all built on the same plan: Each one of an earthworm's segments contains nerves, blood vessels, and excretory organs called nephridia arranged in the same pattern. Many bilaterally symmetrical animals also show a feature known as cephalization, a trend toward 'front-end' development. Some animals with only rudimentary cephalization simply have a distinct front end that leads the way when the animal moves. But in other animals, the front region, or head, has become the part of the body that houses the brain and most of the sense organs. Particularly noticeable in arthropods and vertebrates, cephalization gives active animals the earliest possible information about food, danger, and other aspects of the environment ahead. In comparing two species, anatomists have to be careful to differentiate between homologous structures, which are ones that have evolved from a shared ancestor, and analogous structures, which have developed from different origins. Homologous structures are built on the same underlying plan. A human arm, a bat's wing, and a whale's flipper look quite different from the outside, but the bones inside reveal that these limbs all have the same basic structure. Analogous structures, by contrast, often look similar, but their similarities are only skin deep. A fish's tail fin and a whale's flukes are analogous structures--they look similar from the outside and perform similar functions, but their underlying structures are quite different. Homologous structures are evidence that two species have a shared ancestry. However, analogous structures most often indicate that two unrelated species evolved in a similar environment where both developed structures to perform the same function. IV ANIMAL BODY SYSTEMS A complete anatomical study probes more than a dozen different body systems, from the skeletal and muscular systems, which support and move the body, to the nervous and sensory systems, which enable an animal to interact with its surroundings. To anatomists interested in evolutionary relationships, the underlying structure of each system is often more significant than the exact size or shape of its parts. Evolution changes the individual parts of a system more rapidly than the underlying pattern of how a system or animal is put together. Thus, such underlying patterns often remain intact, providing clues to how species are related. A Integumentary Systems An animal's integumentary system is the external covering that shields its body from the outside world. In addition to protecting the animal from physical damage, it can help the animal prevent loss of body heat or water. The integumentary system is particularly important for animals that live on land because air can quickly dry out and kill living cells. Simple invertebrates, such as sponges and cnidarians, typically have an outer body covering that is just a single cell thick. More-complex animals, including annelid worms, nematodes, and arthropods, are often protected by a nonliving outer layer called a cuticle. In worms this outer layer is thin enough to be flexible, but in arthropods it forms a rigid case around the entire animal. Instead of a cuticle, vertebrates have a multilayered tissue called skin. Although skin sometimes feels soft, its layered growth makes it much tougher than it may seem. In most land-dwelling species, the outermost layer of the skin, called the epidermis, is covered by a thin sheet of dead cells that acts as a weatherproof barrier. These dead cells are constantly worn away, but new cells from the epidermis below rapidly replace them, so the skin never wears through. Underneath the epidermis is the dermis, an elastic layer that contains nerves and blood vessels. Beneath the dermis is the subcutaneous layer, which often contains deposits of insulating fat. During their long history, vertebrates have evolved a wide range of external structures that help the skin to do its protective work. Most fish are covered by scales, which are rough in sharks and rays, but smooth and slippery in most other species. This slipperiness comes from mucus, which is produced by glands in the skin. Mucus makes it easier for a fish to slide through the water, but it also has other uses. At night, a tropical parrot fish rests in a 'sleeping bag' of mucus that makes it harder for predators to attack. At dawn, the fish eats its mucus bag before it swims away. Reptiles also have scales, which serve primarily to help prevent water loss. Birds have scales on their legs and feet, and a few mammals, such as the pangolin or scaly anteater, also rely on this form of body armor. However, birds and mammals have largely abandoned scales in favor of feathers or hair over most of the body. Unlike fish, amphibians, and reptiles, whose body temperature depends on that of the environment, birds and mammals maintain a constant, warm body temperature. Feathers and hair help them retain the heat their bodies generate. Feathers are essentially modified reptilian scales, while hair grows from a follicle within the skin. Although feathers originally evolved to retain body heat, they later developed an additional use in flight. The only major group of vertebrates with bare skin is the amphibians. Although amphibians lack the protection afforded by an outer covering of scales, feathers, or hair, they use their skin to breathe, unlike other vertebrates. Most fish scales are made of bone, but scales in other animals, as well as feathers and hair, are made of a tough and versatile protein called keratin. Keratin is packed into the dead cells on the surface of skin, and also makes up much tougher structures, such as nails, claws, and horns. These structures grow throughout an animal's life. In Asian water buffalo the horns can reach a length of over 1.5 m (5 feet), making these the largest horns in the world. B Skeletal Systems A skeleton is a framework that supports an animal's body and that helps the animal move by giving its muscles something to pull against. Most skeletons are made of hard materials, although the simplest type, called a hydrostatic skeleton, is found in animals that have no hard body parts at all. Hydrostatic skeletons work by pressure, and they need two main components to function: a body cavity that is completely filled with fluid, and a body wall that contains wraparound sheets of muscle. The fluid pushes outward against the body wall, helping maintain the animal's shape. When the muscles in the body wall contract, fluid is forced into other regions of the animal's body, much as squeezing a balloon filled with water causes it to change shape. This process enables an animal with a hydrostatic skeleton to move. Hydrostatic skeletons are common in aquatic animals, such as jellyfish, sea anemones, and tunicates, and are also found in some small land-dwelling invertebrates, such as earthworms and onychophorans (also known as velvetworms). But although this kind of skeleton works well in water, it is not strong enough to support large animals on land--a fact demonstrated by the way jellyfish collapse when stranded out of water by the tide. Hard skeletons enable large animals to counteract the pull of gravity. These skeletons are of two main types. An exoskeleton supports the body from the outside and doubles as a protective barrier, while an endoskeleton supports the body from within. During the course of evolution, animals have created these frameworks from a range of different building materials, including a glasslike material called silica, various calcium-containing compounds, and a tough, waterproof carbohydrate called chitin. Exoskeletons are commonly built from calcium compounds, especially in sea-dwelling animals. Corals, simple invertebrates that are related to jellyfish, build their cases out of calcium carbonate; in fact, a coral reef is really the skeletons of millions of simple animals. Mollusk shells are also made of calcium carbonate, which is secreted by an area of the body surface known as the mantle. But the most complex exoskeletons by far are formed by arthropods. An arthropod's skeleton is built of curved or tubular plates, which hinge against each other at flexible joints. The skeleton completely covers the outer surface of the body, including the eyes, antennae, and feet, but its thickness varies from place to place, so that it provides exactly the right amount of support and protection for each part of the body. Skeletons like these allow arthropods to run, jump, swim, and fly. But these skeletons have one major disadvantage: They cannot keep growing once they have been formed. For this reason, as an arthropod grows it must periodically molt, or shed its exoskeleton, growing a new, larger version in its place. Unlike an exoskeleton, an endoskeleton can reach a large size without becoming too heavy and cumbersome to carry around. Endoskeletons have a wide variety of different structures and are built from many different materials. Sponges are supported by an internal network of spicules, small, pointed structures made of silica or calcium compounds. Echinoderms have internal skeletons made of small, chalky plates. Vertebrates are the only animals that have internal skeletons made of bone. Bone is a living tissue that grows in step with the rest of the body. The earliest vertebrates lived in water, but as they emerged onto land, their skeletons adapted to the increased effects of gravity and the demands of moving about on legs. In general, their bones became denser and stronger, and in dinosaurs and some extinct mammals the bones reached colossal sizes. But not all groups of vertebrates have followed this trend. To help them stay aloft, birds have jettisoned as much surplus weight as possible, evolving hollow, air-filled bones. Their skeletons typically make up about 4 percent of their body weight, compared with 6 percent for mammals of a similar size. Frigate birds have carried this weight saving to an extreme: they have a wingspan of 2.1 m (7 ft), but their skeletons weigh just 115 g (4 oz). C Muscular Systems Nearly all groups of animals, including relatively simple animals such as jellyfish and flatworms, have muscle cells, which are specialized to move parts of the body. Muscles can move an entire animal--a process called locomotion--and they play an important part in the body's internal life, helping other systems to function. Muscle cells, also known as muscle fibers, are usually arranged in bundles or sheets. They work by contracting, and they are triggered into action by nerves, hormones, or their own in-built rhythms. Some muscles relax almost immediately after they have contracted, while others can stay contracted for a long time. A notable example of this extended contraction is seen in clams and other bivalve mollusks, which use muscle power to keep their shells tightly shut at low tide. Once the shell-closing muscles have contracted, they can remain locked for hours without tiring. In contrast, one of the strangest forms of muscle tissue, known as electroplaque, has completely lost its power to contract. Found in electric eels, torpedo rays, and other electric fish, this kind of muscle acts as an on-board battery pack, generating an electric current. In electric eels it can deliver a 600-volt shock--enough to stun or kill fish nearby. Vertebrates possess three different types of muscle tissue. Skeletal muscles, of which there are over 400 in the human body, are attached to bones and move parts of the skeleton in relation to each other. These muscles are under conscious or voluntary control--that is, an animal decides when to use them. Skeletal muscles are used in running, jumping, lifting, or other movements of the body. A second type of vertebrate muscle, called smooth or visceral muscle, is not voluntarily controlled. Smooth muscle lines many hollow internal structures, such as the blood vessels and intestines, and it changes the shape of these structures when it contracts. Smooth muscle contractions push food through the digestive system and carry out other functions, such as adjusting the diameter of blood vessels to regulate blood pressure. The third type of muscle is cardiac muscle, found exclusively in the heart. Unlike skeletal and smooth muscle, cardiac muscle contracts spontaneously without needing any trigger from outside. This pattern of three distinctive muscle types has endured throughout vertebrate evolution, but the arrangement of muscles has changed in many ways. In fish, which resemble the earliest vertebrates, most of the skeletal muscles fan out from either side of the backbone. This feature is easy to see when a fish has been cooked. Muscle often makes up 60 percent of a fish's body weight, and almost all of the muscles are involved in moving the tail and spine, with very few operating other parts of the body. When vertebrates took up life on land, the down-the-spine muscle plan gradually began to change because more muscle power was needed for moving the limbs. Limb muscles became not only bigger but also longer. Some muscle fibers in a frog's hind legs can be a quarter as long as the frog's body, much longer than any muscle fibers in fish. Another important change came about in the chest, where muscles were needed for breathing. In mammals, this trend eventually led to the development of a diaphragm, a dome of muscle that separates the chest from the abdomen and helps to suck air into the lungs. D Nervous Systems For an animal to survive, the cells that make up its body must function in a coordinated way. In most animals coordination is achieved through two body systems: the endocrine system (described in detail in a later section) and the nervous system. The endocrine system works through relatively slow-acting chemical messengers. The nervous system transmits fast-moving signals through specialized nerve cells or neurons. Nerve cells are never preserved in fossils, so there is no direct evidence of how nervous systems developed. However, living animals show a range of different plans that suggest how these systems might have evolved. The simplest plan is the nerve net, in which neurons are scattered roughly equally over the body. Nerve nets are found in cnidarians and, in a more elaborate form, in echinoderms. In a nerve net, the neurons are more or less identical, and there are relatively few of them. There is little coordination of impulses from different parts of the body. Even so, this rudimentary system permits simple patterns of behavior, such as when jellyfish pull in their tentacles if prodded or extend them if they sense food. Invertebrates with a distinct head have nervous systems more like those of humans. These systems are divided into two parts: a central nervous system and a peripheral nervous system. The central nervous system acts as a coordination center and a main highway for nerve signals. The peripheral nervous system carries signals to and from all parts of the body. In this two-part kind of nervous system, the neurons are specialized and work in different ways. Sensory neurons respond to stimuli from outside the body, while motor neurons trigger responses, usually by making muscles contract. For example, sensory neurons in a bee's eye might pick up information about flowers nearby, and motor neurons might then send impulses to various muscles in order to move the bee toward the food source. Connecting sensory to motor neurons are association neurons or interneurons, which process signals before they are passed on. This kind of nervous system enables animals to behave in complex ways, carrying out what look like purposeful, thought-out movements, such as mating behaviors, strategies for avoiding predators and catching prey, and communication with other animals. However, some invertebrates, particularly arthropods, are not quite as intelligent as they seem. Many of their movements are triggered not by the brain itself, but by ganglia, clusters of neurons positioned at intervals down the body. Even if the brain stops working, these animals will often continue to move, although in an uncoordinated way. In vertebrates, the nervous system is dominated by the brain, which controls and monitors almost all of the body's activities. The spinal cord acts primarily as a relay system, although it can activate some movements on its own. One example is the withdrawal reflex, which makes us pull our hands away from anything painful, such as a hot stove. This reflex occurs so quickly that we are often aware of it only after it has happened. In these situations, if pain impulses had to travel to the brain for processing, a burning injury could result before a message to pull away could travel from the brain to the hands. All vertebrates have a brain with three main parts: the hindbrain, midbrain, and forebrain. During the course of evolution, the relative proportions of these brain regions have altered dramatically, and so have some of the functions that each part performs. The hindbrain, which is responsible for basic, involuntary functions such as breathing, has changed least. However, in birds and mammals one part of the hindbrain, the cerebellum, has expanded to coordinate balance and movement. The cerebellum is particularly important in birds, because flight requires faster decision-making than any other kind of movement. In mammals, the forebrain has undergone an almost explosive expansion. Its folded upper region, called the cerebrum, has become so big that most of the rest of the brain is hidden beneath it. This large mass of brain tissue--the cerebrum makes up 85 percent of the brain's weight--carries out a wide range of tasks, including processing signals from the eyes and ears, triggering voluntary movements, and storing and analyzing information. E Sensory Systems For a nervous system to be useful, it must enable an animal to sense changes in its environment and react to them in an appropriate way. The task of detecting such changes is carried out by specialized cells called receptors, which pass signals on to sensory neurons. Some senses, such as touch, involve receptors that are scattered over the body, while others, such as vision, involve receptors that are clustered together in a particular sense organ. Humans are often said to have five senses, but our sensory abilities, like those of most animals, are actually wider than this. In addition to vision, hearing, taste, smell, and touch, we also have a sense of balance or equilibrium, provided by receptors in the inner ear. This sense makes us aware of movement and the pull of gravity. We have skin receptors that respond to cold and heat, and internal receptors that assess the temperature, pressure, and chemical composition of the blood. Internal receptors also monitor our posture--essential information for any organism that walks by balancing on two feet. Other animals share many of the senses that we have, and some can detect additional factors that we cannot. For example, sharks and rays detect the weak electrical fields that other animals generate, while snakes detect heat given off by their prey. Both of these senses help guide predators toward their prey, allowing the animals to attack in murky water or total darkness. In rattlesnakes, the thermal sense works through a pair of heat-sensitive pits on either side of the head, and these animals can detect a temperature difference of just 0.2° C (0.35° F). During the history of animal life, evolution has produced many designs for sense organs. The simplest light-sensing organs, for example, consist of a bundle of neurons backed by spots of dark pigment. "Eyes" like these, which are found in flatworms, simply tell an animal what direction light is coming from, so that it can either creep toward the light or move away. Image-forming eyes are much more complex and follow one of two basic patterns. Compound eyes, which are found in crustaceans and insects, are divided into hundreds or thousands of small units called ommatidia. Each unit contributes a small part of the complete picture. By contrast, the eyes of vertebrates and cephalopod mollusks have only a single unit with one lens, although the lens can change shape to focus on objects at varying distances. Complex sense organs such as the vertebrate eye take millions of years to develop, but they are soon abandoned if they cease to be useful. Vertebrate species such as cave salamanders that have taken up life in dark places have often lost the use of their eyes. Further back in evolutionary history, an entire sensory system was lost as animals took up life on land. This sensory system, known as the lateral line, consists of a row of sensory pits along each side of a fish's body. The lateral line enables fish to detect pressure waves in water, but has disappeared in land vertebrates. F Endocrine Systems Most animals rely on nerves to coordinate their responses to the world around them. To coordinate internal processes, such as metabolism, growth, and development, animals use chemical messengers called hormones. Compared to nerve impulses, hormones travel through the body and take effect fairly slowly. But their effects also last longer than nerve impulses, shaping events in the body over minutes, hours, or even weeks, rather than mere seconds. Hormones are produced by the endocrine system, a diverse collection of glands that empty into the bloodstream or into other body fluids. Once a hormone has been released, it travels through the body until it contacts its target cell. When this happens, the hormone triggers biochemical changes in the target cell, altering the way the cell works. Although hormones can have far-reaching effects, these chemicals are usually present in tiny amounts. In adult humans, for example, the entire bloodstream contains less than 0.0005 g (0.00002 oz) of thyroxine, the hormone that controls the body's overall metabolic rate. Hormones play an important a role in the lives of invertebrates, but many of the substances involved are still poorly understood. One group of hormones that has been studied in detail is the one that controls growth and molting in insects. In most insects, molting is promoted by a hormone called ecdysone, which is produced by glands in the thorax (the middle part of the body). Molting is inhibited, or prevented, by juvenile hormone, which is produced by glands in the head. The slowly changing levels of these opposing hormones make an insect molt periodically as it grows up. If any of these glands is removed, the control system breaks down. This can either make an immature insect molt too many times, so that it never grows up, or make the insect race through childhood, so that it turns into a miniature adult prematurely. More than 50 hormones have been identified in vertebrates. In addition to the metabolism-regulating hormone thyroxine, other key hormones include insulin, which helps regulate blood sugar; antidiuretic hormone, which adjusts the blood's water content; and growth hormone, which speeds up cell division. These hormones are released all the time. Others come into play only in particular circumstances, or at certain stages of life. For example, epinephrine or adrenaline is a hormone that is released in moments of stress. Unlike most hormones, its effects are almost instantaneous. Release of adrenaline causes the heart rate to increase, as well as other changes that prepare an animal for emergency action when it is faced by danger. Vertebrates share many hormones, although a hormone may have different effects in different species. Thyroxine from a cow, for example, can affect a tadpole. However, instead of regulating the tadpole's metabolism, it triggers the tadpole's metamorphosis into a frog. G Respiratory Systems Oxygen is an essential requirement for all animal life because animal cells need it to break down food molecules and generate energy in a process called cellular respiration. Respiratory systems help animals extract oxygen from the surrounding air or water and enable animals to get rid of the waste gas carbon dioxide. Some animals are able to obtain enough oxygen without any specialized respiratory systems at all. Oxygen simply diffuses through the body surface, eventually reaching all the body's cells. However, this way of obtaining oxygen works only in animals such as flatworms, which have small, thin bodies and low oxygen demand. In larger, more active animals, the body's surface is not big enough to take aboard all the oxygen that the animal needs. Extra surfaces are required, and these are provided by respiratory organs. Respiratory organs are found in both land and water animals, but the physical differences between water and air mean that respiratory systems have evolved in different ways in these two environments. In aquatic animals, the most common respiratory organs are gills. Gills are outgrowths of the body, and the simplest of them, seen in sea slugs and some worms, are little more than tufts that protrude into the water. In other animals, such as bivalve mollusks and fish, gills are much more elaborate, with sets of parallel plates arranged to intercept the water flow. The animal actively pumps water over these plates, which are supplied with many tiny blood vessels to pick up oxygen from the water and carry it to the rest of the body. Gills of this type are extremely delicate, and they are usually hidden away inside shells or behind protective flaps, making them invisible from outside. Even though air contains more oxygen than water, gills rarely work on land. This is because without water for support, the flaps of the gills stick together or collapse. Instead, all land vertebrates, together with an assortment of land-dwelling invertebrates, breathe with the help of lungs. While gills are outgrowths, lungs are infoldings of the body surface. In spiders and their relatives, the folds are arranged like the leaves of a book, giving these respiratory organs the name book lungs. A land snail's single lung is a simple cavity underneath its shell that works passively, capturing enough oxygen to match the snail's sluggish lifestyle. Vertebrates need a much larger oxygen supply for their active lifestyles than lungs built on an invertebrate plan could provide. The air spaces in vertebrate lungs divide many times, creating a spongy tissue containing a dense network of blood vessels. The membrane separating the air and blood is often just two cells--or about 0.5 micrometers (about 0.00002 in)--thick, which means that oxygen and carbon dioxide can easily travel across this barrier. In addition, vertebrates use muscle power to pump air in and out of their lungs. Reptiles and mammals suck air into their lungs and then blow it out again, but in birds the air travels straight through the lungs in a one-way flow. Bird lungs are connected to a set of air sacs that change shape to pump the air, while the lungs stay the same shape. This unique system is an extremely effective way of extracting oxygen from the air, and it enables birds to fly at altitudes that would leave mammals gasping for breath. In the insect world, a very different kind of respiratory system has evolved, based not on lungs but on air-filled tubes known as tracheae. These tubes reach deep into the insect's body from openings called spiracles, supplying oxygen directly to all the internal organs. In small insects, the airflow is completely passive, but in large ones, such as grasshoppers, it is helped by body movements. Insects that live in water also have tracheae, indicating that they originally evolved on land. Some come to the surface to breathe, but others, such as dragonfly and mayfly larvae, have developed gills connected to their tracheae. Oxygen flows through their gills and into their tracheae, and then into their bodies. H Circulatory Systems For an animal's body to work properly, vital substances have to move about within it. These substances include oxygen, carbon dioxide, food, waste products, and hormones, as well as factors used to fight disease. In a few of the simplest animals, the body is small and thin enough that substances can simply diffuse from cell to cell. In other animals, the body is too thick for diffusion to work effectively--cells in the center of the body would starve. In these animals, the task of collecting various substances and delivering them to different parts of the body is carried out by the circulatory system. Circulatory systems have two main components: a fluid that does the carrying, and a mechanism for channeling the fluid around the body. The fluid, generally known as blood, varies enormously throughout the animal world. In vertebrates, the blood contains billions of red blood cells or erythrocytes. These cells receive their color from the pigment hemoglobin, which carries oxygen. Earthworms also have hemoglobin, although it is not contained in red blood cells, while crustaceans and cephalopods have a blue-colored pigment called hemocyanin that carries oxygen. Insects have no blood pigments at all. Their blood is clear or yellow--the red color of the fluids from some squashed insects comes from blood they have eaten, not from their own blood. In insects, blood travels forward along the body through a tube called the dorsal vessel, which contains a long muscular section that acts as a heart, propelling blood along the vessel. Once the blood has left this vessel, it flows back through the body spaces, bathing all the internal organs. In this kind of system, called an open circulation, blood makes up a large percentage of the animal's total weight. The blood flows slowly, sometimes taking over an hour to complete its circuit around the body. Open circulatory systems are found in many invertebrate groups, including crustaceans, snails, and clams. All vertebrates, as well as annelid worms, octopuses, and squids, have an alternative pattern called closed circulation. In a closed circulation, the blood travels within a system of blood vessels. It is pumped out of the heart into a branching network of thick-walled arteries, and it returns to the heart through a network of veins. The arteries and veins are linked by microscopic vessels called capillaries, which bring the blood into close contact with all the body's cells. The capillaries have such thin walls that oxygen and other substances can easily diffuse through them and into the tissues nearby. In this kind of circulatory system, the blood volume is relatively small, but it is under high pressure. As a result, it moves quickly. Human blood, for example, speeds around the entire body in a minute or less. In the simplest vertebrate circulatory systems, seen in fish, blood flows in a single loop, traveling from the heart to the gills, and then on around the body. In other vertebrates, a more complex pattern has gradually evolved. Instead of flowing in a single circuit, blood flows in a double loop, first through the lungs, and then back to the heart before moving on through the rest of the body. The circuit that takes blood to and from the lungs is known as the pulmonary circulation, while the circuit that takes blood around the rest of the body is called the systemic circulation. The advantage of this double system is that the blood receives an extra push from the heart after it has picked up oxygen in the lungs. This extra push causes the blood to flow swiftly and deliver oxygen to the body more effectively. Along with these changes in the structure of the circulatory system have come changes in the anatomy of the vertebrate heart. A fish's heart has two main chambers, while the hearts of amphibians and most reptiles have three chambers. Birds, mammals, and crocodiles have four-chambered hearts. Their hearts work like two hearts side by side, keeping the pulmonary and systemic circulations completely separate. This separation prevents oxygenated and deoxygenated blood from mixing and enables the circulatory system to deliver more oxygen to the body tissues--a crucial feature for sustaining the high metabolic rate characteristic of birds and mammals. I Lymphatic Systems When blood flows through capillaries, some of its fluid is squeezed out and into the surrounding tissues. Without drainage, this fluid would gradually build up, and the blood's volume would steadily drop. In vertebrates, the excess fluid is collected and returned to the blood by the lymphatic system, a collection of thin-walled tubes that extend throughout the body, often shadowing the blood vessels. Unlike blood vessels, which form a continuous circuit throughout the body, lymphatic vessels begin as closed, fingerlike tubes throughout the body's tissues. The fluid they contain, called lymph, is channeled through the system and eventually emptied into veins near the heart. In mammals, lymph is kept on the move by the contraction of the body's muscles, and valves at intervals along lymphatic vessels prevent the lymph from flowing backward. Other vertebrates also have these valves, and they often have lymph hearts as well, which pump the fluid along. Some birds have two lymph hearts, while frogs can have nearly a hundred. In addition to draining the body's tissues, the lymphatic system carries out several other functions. For example, it transports some hormones, and it ferries microscopic globules of fat from the intestines to the bloodstream, which delivers these high-energy particles to the body's cells. But one of its most important secondary roles is in fighting disease. In mammals, this work is carried out in bean-shaped swellings called lymph nodes in which lymph is filtered and any foreign matter is engulfed and removed. J Immune Systems For bacteria and other microorganisms, animal bodies can be ideal places to live. Inside an animal body, microorganisms are sheltered from the physical environment, and the animal body provides a ready source of food. But when microorganisms enter an animal's body and become established there, many can cause disease. To counter the threat posed by such microorganisms, animals have evolved methods of keeping intruders out, and of destroying any that do manage to enter the body. Collectively, these strategies make up the immune system--a battery of defenses so complex that they are not yet fully understood. An animal's skin or body covering is the simplest part of the immune system and the first line of defense against microbial invaders. The body covering helps keep harmful microorganisms out, but it is not germ-free. Instead, it often harbors billions of harmless bacteria. This collection of harmless microbes, known as the animal's bacterial flora, makes it harder for dangerous species to become established because they cannot compete for space or resources with the harmless skin-dwelling bacteria. If any microorganisms breach this outer barrier and enter the body itself, the immune system immediately reacts. Many animals, including all vertebrates, have wandering cells called phagocytes that home in on intruders and engulf them. In humans and other vertebrates, the phagocytes circulate in the blood, reaching the site of any infection by squeezing through the walls of capillaries into the surrounding tissues. Phagocytes act rapidly and are nonspecific, meaning that they respond to any kind of foreign substance or organism in much the same way. Vertebrates also have a much more sophisticated defense mechanism that targets invaders with far greater precision. Known as the adaptive immune system, this mechanism enables the body to 'memorize' the chemical identity of any alien substance. If the same substance appears a second time, the immune system attacks it with much greater speed and efficiency than the first time. The main components of the adaptive immune system are proteins called antibodies and cells called killer T lymphocytes. Antibodies circulate in the blood and help other parts of the immune system recognize and destroy foreign substances. Lymphocytes are found in the blood, lymph nodes, spleen, and thymus gland. The remarkable feature of the adaptive immune system is its ability to target a vast range of alien substances, without targeting the body's own cells. So far, nothing like this mechanism has been found in the invertebrate world, and it is not clear how this elaborate system originally evolved. However, even the very simplest animals show a clear ability to distinguish between their own cells and cells from some other organism. This ability is demonstrated by experiments in which sponges are passed through fine sieves so that their cells are separated from each other. If the cells from two species of sponges are mixed together, they slowly crawl apart, forming small single-species sponges once more. During vertebrate evolution, the antibody system has gradually become more complex and diverse. Fish have just one class of antibodies, all sharing a common biochemical backbone. By contrast, reptiles have two classes, while humans have five. This increasing diversity has probably come about through the biological equivalent of an arms race, as animals and disease-causing organisms each struggle to gain the upper hand. K Digestive Systems Animals need food for energy and for raw materials to build their bodies, but before they can use food, they have to break it down into basic component molecules. This process is called digestion, and it can be carried out in two quite different ways. In one method, known as intracellular digestion, an animal's cells engulf nearby particles of food. Once a cell has swallowed a food particle, it is stored in a fluid-filled compartment called a vacuole, where digestive chemicals break it down. In the second digestive method, called extracellular digestion, the food never enters the body's cells directly. Instead, it is broken down outside the cells, and only the digested products are absorbed. Intracellular digestion is widespread in protozoans but much rarer among multicellular animals. Sponges use intracellular digestion, as do cnidarians and flatworms, although they also use extracellular digestion as well. Most other animals, including all vertebrates, rely on extracellular digestion alone. The great advantage of this method is that it enables an animal to tackle much bulkier kinds of food: In the deep sea, for example, gulper eels sometimes catch and digest animals that are bigger than themselves. The work of breaking down and absorbing food is carried out by an animal's digestive system. The simplest kind of digestive system, seen in cnidarians and flatworms, has just a single opening or mouth that leads to a space inside the body. This arrangement means that after food has been digested, any undigested remains have to be expelled the same way they came in. A much more common plan is based on a hollow tube, called the gut or alimentary canal, that runs right through the body. Food enters the tube through one opening, the mouth, and leftover waste leaves it through another, the anus. A key feature of this system is that it works like a production line: Different parts of the tube carry out different digestive tasks while food is on the move. Mammals show how evolution has adapted this production-line digestive system to deal with different foods. In most mammals, apart from those that eat only insects and plankton, teeth play an important part in collecting food and getting it ready to be digested. Teeth can stab, slice, or chew food to break it into smaller pieces before it goes to the stomach. When teeth are in the form of tusks such as in elephants, they can even be used as instruments for digging or to help bring food closer to the mouth. Once food has been swallowed, it begins its journey through the digestive system. In carnivorous and omnivorous mammals--those that eat meat or that have a varied diet--the first stop is the stomach, where powerful acids and enzymes begin to break the food down. The food then moves on to the small intestine, where more digestive enzymes are added to the mix. Some of these enzymes come from cells lining the intestine, and many others come from the pancreas, one of several organs attached to the alimentary canal. The small intestine absorbs useful substances as the digested food travels through it. Finally, after a journey that may last several hours in larger animals, the undigested waste arrives in the large intestine, where any surplus water is absorbed. After the undigested waste passes through the large intestine, it is eliminated from the body through the anus. In herbivorous, or plant-eating, mammals, digestion is usually more complicated. This is because mammals do not have any enzymes that can digest cellulose, the tough, structural substance that makes up plant cell walls. To survive, they rely on symbiotic microorganisms to do this work for them. Certain hoofed mammals called ruminants store millions of these microbes in the rumen, the biggest part of their three- or four-chambered stomachs and the first stop for food after these animals swallow it. After the microorganisms have had time to break down the cellulose in the food, the animals regurgitate the food and chew it a second time. When the food is swallowed after this second chewing, it continues its journey through the stomach and into the intestines. This roundabout process allows these mammals to obtain more energy and nutrients from their food than they would be able to if cellulose passed through their bodies undigested, as it does in humans. Unlike mammals, birds do not have teeth, which means that they cannot chew. To grind up plant food, they have a gizzard, a muscular stomach with a hardened lining. Gizzards are also found in crocodiles and their relatives, and in some insects. Birds and crocodiles often swallow stones or pieces of grit to help the gizzard do its work. Fossilized remains from New Zealand show that moas, which were among the largest birds that have ever lived and became extinct in the early 19th century, carried over 2 kg (4.4 lb) of these stones, some as big as golf balls. Where stones are hard to find, crocodiles have been known to swallow glass or pottery instead. L Excretory Systems Animals generate two kinds of bodily waste. The first is waste from the digestive system, matter that has traveled through the alimentary canal without being absorbed. This type of waste is fairly easy to eliminate. It simply passes out of the body through the anus. The second type is chemical waste that is generated inside the body itself--substances including carbon dioxide, nitrogen containing compounds, a variety of salts, and surplus water. This internal waste is potentially dangerous because if it accumulates in the body, it can poison living tissue. Animals have developed a variety of strategies for excretion, the process of removing this chemical waste. Carbon dioxide is a product of the chemical reactions living things use to release energy. In very small animals, carbon dioxide seeps out of the body before it has a chance to build up. But in larger animals, including humans, carbon dioxide can quickly cause problems if it is not removed. The work of removing this waste gas is carried out by the circulatory and respiratory systems. In birds and mammals in particular, the level of carbon dioxide in the blood is controlled very carefully. If it rises even slightly, a part of the brain triggers faster and deeper breathing, which speeds up carbon dioxide loss through the lungs. This continues until the normal carbon dioxide level is restored. Nitrogen-containing, or nitrogenous, waste is a by-product of the chemical breakdown of proteins in food. This waste can be highly toxic, and it has to be removed from the body without delay. The nitrogen-containing compound ammonia is often an end product of the breakdown of proteins, so the simplest way to dispose of nitrogen is in this form. However, ammonia is so poisonous that it must be diluted in generous amounts of water to prevent it from causing harm. Aquatic animals generally excrete nitrogenous waste in the form of ammonia, because they can easily obtain enough water from the environment to safely flush this compound away. Land animals have evolved ways of converting ammonia into less dangerous compounds, such as urea and uric acid. Urea is water-soluble, and mammals excrete this substance in their urine. Uric acid is a much less soluble compound that can be disposed of in a semi-solid form. Birds, reptiles, and insects all excrete nitrogencontaining waste in the form of uric acid. This characteristic is linked to the fact that these animals lay shelled eggs: unlike urea, uric acid can be stowed away in an egg without poisoning the animal developing inside. In vertebrates, nitrogenous compounds, together with salts and other waste products, are removed from the body by the kidneys. Kidneys work like filters, removing waste and water from the blood, then returning most of the water to the bloodstream. In mammals, this waste is expelled via the bladder. In birds and reptiles, the waste is emptied into the cloaca, a chamber that serves as an exit point for the digestive and reproductive systems as well as the excretory system. Salt can also escape from the body in other ways. Some mammals, including humans, excrete salt in sweat, while seabirds and crocodiles have special glands that exude it in a salty fluid. In seabirds, these glands are behind the nostrils, and in crocodiles they are at the back of the tongue. Kidneys originally evolved to control the body's water balance, and this is still an important part of their function. Animals that live in dry habitats have developed highly efficient kidneys that keep water loss to a minimum. The waste substances in human urine are usually about 4 times as concentrated as they are in the blood. By contrast, in desert animals, such as kangaroo rats, waste substances can be over 15 times as concentrated. This means that a kangaroo rat uses much less water to dispose of the same amount of waste. Invertebrates do not have kidneys, but they do have organs that work in similar ways to remove and excrete waste from the bloodstream. These organs include nephridia in earthworms and Malpighian tubules in insects. The nephridia of earthworms are arranged in pairs, with one pair per body segment, and they open to the outside of the body through microscopic pores. Malpighian tubules attach to an insect's gut and empty directly into it, so that nitrogenous wastes leave the body through the anus. Insects that have a high-protein diet, such as blood-sucking flies, must dispose of a large amount of nitrogenous waste, and their Malpighian tubules are particularly well developed. While land animals must try to conserve water, surplus water can be a serious hazard for some kinds of freshwater life. Their body fluids contain more salts, proteins, and other substances than the water around them does, so water is driven into their bodies by osmosis. This process moves water molecules across cell membranes until the concentration of dissolved substances is the same on both sides of the membrane. Without a water disposal system to counteract osmosis, these animals would run the real risk of exploding. Protozoans get rid of surplus water by using contractile vacuoles, internal reservoirs that fill up with water and then pump it out of the cell. Freshwater sponges also bail out water in this way. Fish, meanwhile, use their kidneys to filter excess water from the blood. Freshwater fish never drink, except when swallowing food, but even so they have to excrete water all the time. M Reproductive Systems Reproduction is the most important task that any animal undertakes. Reproduction ensures that the species will continue to survive even though individual animals grow old and die. It also gives species an opportunity to increase their numbers and to evolve as time goes by. The simplest method of reproduction involves a single parent and no specialized body parts at all. The parent divides into two or more similar pieces, each of which becomes a new animal. This method of multiplying is known as asexual reproduction. It is carried out by some simple invertebrates, including sponges, sea anemones, and flatworms, but it reaches a high point among ribbon worms. These worms periodically disintegrate into a dozen or more sections, each of which grows a new head and tail. Although it is a simple, reliable way of multiplying, asexual reproduction has one very important disadvantage. Only one parent is involved, so all the offspring are genetically identical, both to each other and to the parent. This lack of genetic variability means that the offspring are all equally vulnerable to disease or other hazards. If faced with some disease or change in the physical environment, all the offspring may die. This is the reason why most animals reproduce sexually, a process that ensures genetic variability in the offspring. If the offspring have different combinations of genes, it is more likely that at least some of them will have traits that enable them to survive a disease or other hazard in the environment. Sexual reproduction is a much more complex process than asexual reproduction and requires two partners. For sexual reproduction to occur, specialized sex cells or gametes are also needed. These cells are made in sex organs called gonads. Male sex cells, or sperm, are produced in testes, while female sex cells, or eggs, are produced in ovaries. Sex cells have half the number of chromosomes, the units that contain heredity material, found in normal body cells. During sexual reproduction, an egg cell joins with a sperm in a process called fertilization, creating a cell with the normal number of chromosomes. The cell then divides to become an embryo and eventually develops into a fully formed animal. The combination of chromosomes from the sperm and egg gives the offspring a new and unique genetic makeup, different from that of either parent. Egg cells are typically large compared to normal body cells. The size disparity is particularly marked in reptiles and birds. In these groups, an egg cell sometimes weighs over a billion times as much as a body cell. Sperm cells, on the other hand, are little more than packages of genes, typically powered by a hairlike flagellum that pushes them along. In many animals, sperm are produced in much larger quantities than eggs are. Fertilization must take place in watery surroundings because otherwise sex cells would soon dry out and die. For animals that live in water, this requirement poses no problems. Most of them release their sex cells into the water, so fertilization takes place outside their bodies. On land, almost all animals use internal fertilization, in which the male introduces his sperm directly into the female's body. This need for direct physical contact has generated a vast range of complex patterns of behavior. The male and female have to locate each other, and each has to demonstrate suitability as a partner. Elaborate courtship rituals have evolved to defuse an animal's instinctive fear of being approached, and in animals that pair up for life, ritual behavior maintains the bond between the two partners (see Animal Courtship and Mating). Most animals are either male or female, but this is not always the case. Many earthworms and snails are hermaphrodites, which means that each worm has both male and female sex organs (see Hermaphroditism). The advantage of this system is that any two partners can mate. To reproduce, an earthworm need not find another earthworm of a specific sex--any earthworm will do. In some animals, particularly sap-sucking insects such as aphids, females are able to produce young without having their eggs fertilized by a male. This method of asexual reproduction, called parthenogenesis, allows the animals to boost their numbers very quickly when environmental conditions are beneficial. However, few animals that are capable of parthenogenesis rely solely on this way of reproducing. Most also have a sexual phase in their life cycles, which creates genetic variety in their young. Some animals, including humans, hyenas, and domesticated cattle and pigs, breed all year round. Most, however, breed in step with the seasons, so their reproductive systems are used only at particular times of the year. In many animals, seasonal changes trigger the release of hormones that bring the reproductive system into action. Hormones may trigger mating behavior, such as territorial behavior in males or nesting behavior in females. They may also trigger other changes in a female that signal to males that she is fertile, such as the release of pheromones or the genital swelling seen in some female primates. One of the most dramatic effects of reproductive hormones is seen in male starlings. In these birds, the testes can grow 200 times bigger at the onset of the breeding season, shrinking again once the season is over. By shutting down the reproductive system when it is not needed, a starling sheds surplus weight, saving energy when it flies. V HISTORY OF COMPARATIVE ANATOMY Knowledge of anatomy began in prehistoric times, when people cut up carcasses of animals they hunted, fished, or herded before cooking them. Primitive artists made crude drawings of animals, such as the images preserved in cave paintings, but very little of this ancient knowledge was recorded in writing. The ancient Egyptians probably had some knowledge of the internal anatomies of humans, cats, and other species because they mummified these animals. In the practice of mummifying, the Egyptians removed the internal organs from a dead body and filled the internal cavities of the body with materials that retard decay . The first scholar to produce a large body of writing on comparative anatomy was the Greek philosopher Aristotle, who described and classified about 540 different kinds of animals during the 300s BC. Most other early writings on anatomy dealt primarily with the human body. However, much of the information in these writings was gathered by dissecting animals, so the first writers in human anatomy were, in fact, comparative anatomists. Early anatomists relied on animal dissections in part because the human body was held to be sacred by many ancient peoples, and its dissection was forbidden by law. Although the Greeks began to ease many of these restrictions after about 400 BC, much knowledge of human anatomy was still gleaned from dissections of domestic animals and monkeys. The preeminent anatomist of the ancient world, the Greek physician Galen, probably never dissected a human body. He dissected various domestic animals, monkeys, apes, and even some exotic species killed in the gladiatorial ring. His writings remained the primary authority on human anatomy for nearly 1,500 years, until the Belgian anatomist Andreas Vesalius pointed out that many of Galen's observations on human anatomy were inaccurate because they were based on animal dissections. The Renaissance in Europe (14th to 16th centuries AD) was a period of rapidly increasing knowledge about human anatomy, but some influential scientists continued to be interested in comparative anatomy. English physician William Harvey, best known for his studies on the circulation of the blood, also dissected many animals and advocated the study of comparative anatomy. The term comparative anatomy was first used by English scientist Nehemiah Grew, who published a book in 1681 describing the anatomy of stomachs and intestines in several different species. During the 18th century, knowledge of comparative anatomy advanced rapidly. The French naturalist Louis Jean-Marie Daubenton compared the anatomies of many different animals in a section of Buffon's Natural History (a 36-volume work published between 1749 and 1789 that contained observations about the mineralogical, botanical, and zoological characteristics of the Earth). This section of the Natural History is today considered the first extensive work in comparative anatomy. During the 19th century, comparative anatomy studies helped British scientist Charles Darwin to develop the modern theory of evolution. On a voyage to the Galápagos Islands off the western coast of South America, Darwin saw more than a dozen different species of finches living on various islands. All the finches were similar in size and in their dull, blackish or brownish gray coloring, but their beaks varied widely in size and shape. These similarities and differences suggested to Darwin that the various finch species might be related to one another and that they had all arisen from the same ancestral species. Around the same time, modern concepts of comparative anatomy were developing from the work of many great zoologists. Richard Owen, a British biologist known for his studies of the fossil birdlike dinosaur Archaeopteryx, published the third edition of his Comparative Anatomy in 1871. He also developed the concepts of homology and analogy. Thomas H. Huxley, another British biologist, published his Comparative Anatomy of Vertebrated Animals in 1871. He also established the modern concept of the evolution of the vertebrate skull. German biologist Ernst H. Haeckel contributed to the knowledge of the three germ layers that are found in the early embryos of most animals and develop into the organs of adults. He also established the biogenetic law, which states that during their development from fertilized egg to adult, animals pass through stages that recapitulate their evolutionary development. Although it is now known that this law does not hold absolutely (Haeckel constructed evolutionary trees based entirely on embryology that are now known to be false), Haeckel's idea has remained profoundly influential. VI RECENT DEVELOPMENTS Anatomical research constantly refines our knowledge of how animals are related. Until recently, anatomists relied almost entirely on the evidence of physical features to understand evolutionary relationships, but today they use information from DNA as well. This biochemical evidence has helped to answer several questions about animal evolution. For example, in the late 1980s some scientists put forward a theory that large fruit-eating bats evolved separately from other bats. However, DNA evidence suggests that all bats evolved from primitive insect-eating ancestors, contradicting that theory. The explosive growth in molecular biology has also increased our understanding of how animal bodies develop, and how cells and tissues become specialized, a process known as differentiation. Differentiation has been studied in meticulous detail in one particular animal, a tiny, transparent nematode worm called Caenorhabditis elegans. Scientists have also identified genes in this animal that control the timing of differentiation in separate groups of cells. Another interesting discovery from this research is that cell death is a normal part of forming the body's organs. In Caenorhabditis elegans, over 100 of the animal's cells are programmed to die before the adult body is complete. In 1995, three biologists--Edward B. Lewis and Eric F. Wieschaus of the United States, and Christiane Nüsslein-Volhard of Germany--were awarded the Nobel Prize in physiology or medicine for their discovery of master genes that control the position of different body parts. If one of these master genes is defective, the wrong kind of body part may develop, or the same part may be duplicated in several different places. Their work was originally carried out on fruit flies, but it has since been discovered that similar master genes occur in a wide range of animals, including nematode worms, frogs, and humans. Many anatomists believe that these genes may turn out to play an important part in animal evolution. Contributed By: David Burnie Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.