Comparative Anatomy

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.

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain