All organisms, whether plant or animal, must reproduce, grow, breathe, eat, excrete, and be able to respond to their environment. Various systems in animals’ bodies accomplish these tasks. These systems may vary from species to species, but the chemical processes involved in the activities are usually the same.
Differences in anatomy, or arrangement of limbs and organs, have arisen for several reasons. One is that for early aquatic animals to evolve into more advanced forms that could live on land, various physical features had to be modified. For example, legs and feet replaced fins; and lungs, which enabled animals to get oxygen directly from air, replaced gills.
One of the most important anatomical developments in animals is segmentation, first seen in the earthworm, in which the body is divided into ringlike segments. The more primitive animal phyla lack segmentation, which occurs in all higher groups. In the more advanced animals, such as vertebrates, segmentation occurs only in the embryo stage.
Three basic types of support systems, or skeletons, exist in the animal kingdom: the hydrostatic skeleton, the exoskeleton, and the endoskeleton. Some invertebrates, such as jellyfish, worms, and sea stars, have a skeleton composed of a simple, fluid-filled cell called a hydrostatic skeleton. These animals move by contracting muscles, which shift water held inside the body, bringing about a change in the animal’s shape or position.
Other invertebrates, such as insects and crustaceans, have a hard outer case shell called an exoskeleton. The exoskeieton, which forms from secretions of the animal’s skin, is segmented and bears jointed legs. The muscles that enable these animals to move are attached to the inside of the skeleton.
Vertebrates have an endoskeleton, which is a hard internal support system made of bone. Muscles fixed to the end of each bone work in pairs, one contracting while the other relaxes, enabling vertebrates to move. The joints of the vertebrate body are enclosed in a synovial capsule and are lubricated by synovial fluid.
The basic structure of an animal also determines how it will grow, and the growth patterns of animals differ greatly. Because an exoskeieton does not grow, insects and crustaceans must molt, or shed their exoskeleton periodically, as they outgrow it. Mammals have special growth centers near the tips of their long bones, where the cell division that enables growth takes place. In addition, mammals’ skull bones are separated by cartilage, which allows the skull to grow. Growth stops when the bones meet.
Animals also display a wide variety of adaptations that enable them to move about. Mammals, birds, and many reptiles and amphibians have legs with feet that enable them to walk on land. Other animals can crawl without legs. Some invertebrates, such as planarians and flatworms, slide along by moving tiny hairlike structures called cilia. Snails coat the ground with a sticky fluid and then glide along on a muscular organ called a foot. Snakes use their muscles to bend their bodies back and forth, enabling them to slither along the ground.
Animals also have ways to get around in the air and water. Insects, birds, and bats have wings that allow them to fly. Many animals that live in water, such as fish and whales, have well-developed fins and tails that they use to propel them through the water. Other aquatic animals, such as jellyfish and squid, swim by jet propulsion. Many birds can dive and swim by using their feet and wings as paddles or oars.
The chemical processes that result from gas exchange are necessary to the survival of most animals. These processes take place within the respiratory system. Respiratory organs take a variety of forms. But from the simplest to the most complex, they are mainly concerned with two processes: external respiration, which involves the intake of oxygen and its transport to each cell in the body, and internal respiration, the chemical processes that use the oxygen.
External respiration finishes when the oxygen crosses the respiratory surface, the place where the exchange of gases takes place, and enters the cells. The respiratory surface varies from animal to animal, but it has certain features in common: it is moist and thin (normally one cell thick); it is permeable, allowing substances in solution to pass through it; and in some animals, it is supplied with blood vessels or some other means of transporting oxygen to the cells. Oxygen enters jellyfish and related animals through the surface cells of the entire animal, so that the whole surface of the animal is its respiratory surface.
The moist skin of the earthworm also acts as the respiratory surface, but its body is too thick for the oxygen simply to diffuse from the outer layer to the inner cells. To transport dissolved oxygen to ail parts of the body, the earthworm has a blood circulation system.
In land-dwelling arthropods, such as scorpions, spiders, and insects, a series of air tubes called tracheae runs from the outer surface into the body and ends in special areas of gas exchange called alveoli. Each alveolus is kept moist so that oxygen can dissolve into the water and thus leave the alveolus to diffuse into the body cells. Aquatic invertebrates with an exoskeieton, such as crabs and lobsters, and aquatic vertebrates, such as fishes, have a gill system that acts as a respiratory surface, extracting oxygen from the water.
The lungs of land vertebrates contain air tubes, alveoli, and a supply of blood vessels to carry the oxygen to other parts of the body. In mammals, a ribcage protects the lungs, and a muscular sheet called the diaphragm separates them from the rest of the internal organs. The ribs and diaphragm act together to pump a continuous stream of fresh air into the lungs. Air entering the lungs dissolves in the film of water in the alveolus chamber and diffuses through the chamber wall into the blood vessels. A red substance called hemoglobin in the blood transports the oxygen to the cells.
The nervous system
Every animal has a nervous system, which allows it to sense and interact with its environment. Jellyfish and sea anemones have identifiable nerve cells that form a nerve net across the surface of their bodies, but they lack a central nervous system and respond generally to stimuli such as light and dark and hot and cold.
In worms and arthropods, some nervous tissue is concentrated at the front of the body. In these animals, a primitive brain joins a thick ventral nerve cord that runs the length of the body. Smaller nerves branch from it to other parts of the body, and so a basic informationcollecting system is formed.
Vertebrates have a specialized complex nervous system composed of a brain and a spinal cord, which is protected by the spinal column. The brain is connected to the eyes, nose, ears, and other parts of the head by a series of cranial nerves. Spinal nerves connect the rest of the body to the spinal cord. The warm blood of mammals and birds provides the stable internal environment necessary for the development of their large brains.
The eyes of animals provide evidence of the evolution of sensory organs. The eyes of some crustaceans and worms are sensitive only to light—these animals have simple clusters of photoreceptors, called ocelli, which guide them toward and away from light. In higher animals, the sensory cells are so organized that an image forms inside the eye. Lower vertebrates such as reptiles, amphibians, and fish have lateral vision, in which each eye—one on each side of the head—forms a separate two-dimensional image of a scene. But mammals and birds of prey have three-dimensional stereoscopic vision. Their eyes are directed forward so that a single image of a scene is formed.
Chewing and digestion
Most animals merely swallow their food, making no attempt to chew it. In some groups, such as birds and some insectivores, tiny stones in the stomach help break the food down so that enzymes can digest it further. Most vertebrates use their teeth only to hold food before swallowing it whole.
Mammals, however, must begin the digestion process in the mouth by chewing. This helps them get the maximum nutritional value from the food they eat to supply the increased energy needed for warm-bloodedness. Mammals have developed different types of teeth depending on their diet. Herbivorous, or plant-eating, animals generally have long jaws with a set of flat-topped, grinding cheekteeth to break up tough plant material. Carnivorous, or meat-eating, animals have sharp teeth, or canines, which they use to kill their prey, along with slicing cheek teeth, or carnassials, which they use to cut the meat into swallowable pieces. Omnivorous mammals, which eat both meat and plants, have a combination of flat-topped cheekteeth with sharp, cutting edges and incisors for biting.
In all animals, whether simple or complex, enzymes convert food to its simplest form. This process is called digestion. Jellyfish, sea anemones, and their relatives take food in through the mouth—the only opening in the body. The lining of the internal cavity then secretes enzymes that digest the food material, and special cells inside the cavity engulf the digested food. Unused food is rejected from the body via the mouth.
Most other animals have a tube called the alimentary canal that runs from the mouth to an opening called the anus in the rear. The food enters the tube via the mouth, which opens into the buccal cavity, where the digestive process begins. The food then passes along a muscular section of the tube, pushed by wavelike contractions called peristaltic waves, until it reaches the stomach. Most of the digestive process occurs in the stomach or an area close to it, such as the intestines. Once digestion is completed, the body absorbs the useful materials, and the waste products are passed out of the body via the anus.
In addition, vertebrates and some other animals have specialized glands that produce secretions that aid the digestive process. In mammals, for example, the liver produces bile, and the pancreas releases digestive enzymes into the small intestine. Food broken down by enzymes is absorbed into the blood and taken to the liver, which begins the process of eliminating any unnecessary chemicals.
Most animals also contain beneficial microorganisms that live in their digestive systems. Some animals carry on symbiotic relationships, in which another organism lives within or on the body of another for the mutual benefit of both. For example, single-celled organisms called protozoa live in the guts of some termites and break down the wood that they eat.
Excretion is the process by which animals eliminate waste from their bodies. In many simple organisms waste passes out via the body wall. In larger animals, however, special organs carry on this process.
The circulatory and respiratory systems in vertebrates remove carbon dioxide and water from the body, and the kidneys filter out excess chemical substances brought to them by the blood. The toxic substance ammonia is the end product of metabolism. A great deal of water is required to flush ammonia out of the body. Aquatic animals such as fishes, for which water loss is not as great a problem, excrete ammonia directly. Land animals convert ammonia into less toxic urea and uric acid, which are then collected in the bladder, mixed with a smaller amount of water, and are excreted in the form of urine. Some animals, such as insects and land snails, which need to stay moist by keeping water in their bodies, excrete dry uric acid crystals.