All animals are made up of various parts. Their smallest fully functional parts are cells, which are shaped in different ways according to the tasks that they carry out. Groups of similar cells are organized into tissues, and tissues are grouped together to form organs. Organs themselves are linked to form organ systems, which carry out all the processes essential for survival.
The structure of these systems varies widely between one type of animal and another, and also between animals that live in different ways, but the work they do is the same.
Animals have up-to dozen separate body systems. In many species, the muscular and skeletal systems makeup a large portion of the body’s total weight, while the integumentary system – the parts that form the “outer cladding” protect the body from physical damage and, in terrestrial animals, from the threat of drying. Two key systems enable animals to obtain energy from food. The digestive system breaks food down so that it can be absorbed, and the respiratory system delivers oxygen to the body’s cells so that food substances can be “burned” and their chemical energy released.
The respiratory system also removes carbon dioxide a potentially toxic product formed during the production of energy. In many animals, including all vertebrates, oxygen and carbon dioxide are carried by blood in the circulatory system. Carbon dioxide is usually breathed out; other kinds of dissolved waste are removed before they have chance to build up, by a separate excretory system. Animals use 2 different systems to coordinate their bodies, and to react to their surroundings.
The nervous system deals with anything that needs a fast response processing the information gathered by sense organs. In all animals, it triggers inbuilt or instinctive behavior, but in some, especially vertebrates, it also stores information, allowing animals to adapt their behavior according to their past experience. The endocrine system works in conjunction with the nervous system, releasing hormones, or chemical messengers, that help the nervous system to coordinate long-term processes. Finally, the reproductive system carries out the most important task: producing young. Unlike other body systems, it often functions only during a set season, and then only in mature animals.
SKELETON AND SUPPORT
Animals need to keep the shape of their bodies stable. Many invertebrates achieve this without any hard body parts at all. Instead, they rely on the pressure of internal fluids to keep their bodies firm in the same way that air stabilizes a tyre. This system, called a hydrostatic skeleton, works well on a small scale. But in larger animals, particularly land ones, this type of skeleton is often not strong enough to support the body’s weight. Animals have developed 2 quite different solutions to this problem: external shells and body cases; and internal skeletons, typically made of bone. Shells are made up of one or 2 parts, and they grow in step with their owner. They can be seen in marine animals called brachiopods, or lampshells, but they are most highly developed in bivalve molluscs, which can grow shells over 1 m (31/4 ft) across. Body cases are more complex than shells: they consist of a large number of separate plates that meet at flexible joints. They are a characteristic feature of arthropods – a huge group of invertebrates that includes insects, crustaceans, and arachnids. These case or exoskeletons, cover the entire body and includes structures as strong as a crab’s pincers or as delicate as a butterfly’s antennae. Unlike shells, body cases cannot grow, so periodically they have to be shed and replaced. Internal skeletons (endoskeletons), made of bone and/or cartilage, provide support from within. Found only in vertebrates, they have
MUSCLES AND MOVEMENT
Muscles work by contracting. This means they can pull but not push. In most case, they are arranged in pairs or groups that pull, in opposing directions when one muscle or muscle group, contracts, its partner is brought back to its normal resting shape. Muscles make animals move in different ways. In animals without limbs, such as earthworms and jellyfish. they work to change the body’s shape.
In earthworms, opposing muscles alternate shorten and lengthen the animal’s segments so that it can creep through the soil. In limbed animals, one set of muscles pulls the limb down or back, while the other lifts it up or forwards. As well as making animals move, muscles serve other purposes. They force food through the digestive system (peristalsis) and pump blood around the circulatory system. Unlike most other muscles, the heart muscle has a built-in rhythm that keeps it contracting throughout an animal’s life.
Animal cells are easily damaged. To protect them from injury and disease, animals have body coverings, most of which consists largely of non-living matter. Mammalian skin is covered by dead cells, while insect body cases are covered by a hard substance called “chitlin” and waterproof wax.
In many cases these protective layers are themselves protected: mammals often have a coat of fur, while many other animals have scales. Some of these extra coverings have developed additional uses. Soft features and fur help to retain body heat, while extra -strong feathers are used in flight. Colors or patterns act as camouflage or help animals to recognize their own kind.
For small and thin animals, obtaining oxygen is a simple matter because it seeps into their bodies from outside. At the same time, carbon dioxide escapes in the other direction. For larger animals, respiration is more complex. In relative terms, they have a much greater volume than surface area, so there is less room for gases to move in and out. To breathe, they rely on respiratory organs-structures that effectively pack a large surface area into a small amount of space. In aquatic animals, gills are the most common respiratory organs. Typical gills consist of thin, flat, or feathery surfaces that bring blood into close contact with the water outside.
However, most gills do not work in air because — out of water — their surfaces collapse and stick together. Land animals therefore have hollow respiratory organs that carry air deep inside their bodies. In insects, these organs are tubes, called tracheae, which divide into extremely fine filaments that reach individual cells. In land-dwelling vertebrates, the organs are lungs — air filled chambers surrounded by a network of blood vessels. Muscles make the lungs expand or contract, sucking air in or blowing it out.
Nerve cells, or neurons, are the animal world‘s equivalent of wiring. Neurons conduct brief bursts of electricity, known as impulses, which carry information from sense organs or make muscles contract. Corals and other simple animals have a network of nerves scattered throughout their bodies. But in most animals, the nervous system converges on the brain.
Some animal senses, such as touch, operate through nerve endings scattered all over the body. A similar sense, which works internally, tells animals about their posture. The most important senses — vision, smell, and hearing — work through organs that form some of the most elaborate structures in the body. Vision is essential for many animals and eyes show a wide variety of designs. At their simplest — for example in snails they do little more than distinguish between light and dark. In many animals, particularly arthropods and vertebrates, they focus light onto large numbers of nerve cells, building up a detailed image of the surroundings. In vertebrates, these eyes have a single lens, which throws light onto a “screen”, or retina.
In arthropods the eye has up to 25,000 separate compartments, each with its own lens system; these compound eyes create a mosaic-like image and are especially good at detecting movement. Mammals are the only animals with prominent ear-flaps. Vertebrates’ ears are always on the head, but in some animals, they are positioned elsewhere.
Most grasshoppers and crickets have ears on their abdomen or legs. Organs that detect taste and smell can also be in a variety of positions. Like ears, they can be used in communication, as well as for avoiding danger and finding food. Many animals have senses that are more acute than those of human beings, and some can sense things that humans cannot. For example, most fish can sense pressure waves in water, and many can detect weak electric fields. Some snakes can see warmth, enabling them to attack warm-blooded prey in total darkness.