Classification and Characteristics of Amphibians

There is an interesting group of vertebrates called amphibians. The name comes from Greek words that mean having two lives. During their life cycle they live in two worlds – the world of water and the world of dry land. Biologists classify the 2,500 living species of amphibians into three orders. Tailless amphibians, such as frogs and toads, Rana, make up the order Anura. Anura means “without a tail.” Salamanders, Triturus, Proteus and other amphibians with legs and tail are placed in the order Urodela. Urodela means “visible tail.” Legless amphibians called caecilians make up the other Apoda, a name that means “without legs.”

Amphibians range in size from a Cuban frog only 1.2 cm long to a giant Asian salamander 160 cm long. Amphibians live on every continent except Antarctica but they are most abundant in the tropics. Most amphibians live part of their life in water and part on land. They usually live in water during the first stages of life. As adults, many kinds live on land. Even as adults, amphibians must be in or near water or be in a place that is very humid. They must keep their skin moist in order to survive. They do not drink. They take the water they need through their skin.

Amphibians adapt for living on land. One example is their mode of respiration. Most adult amphibians have internal lungs rather than external gills. These lungs are contained in the chest cavity and are constantly moistened by water condensed from air and by body fluids. Amphibians have simple saclike lungs that are not as efficient as the lungs of other land vertebrates.

Like fishes, amphibians are cold-blooded; they have no way of maintaining a constant internal body temperature. They remain active as long as temperatures are favorable for movement.

Frogs and Toads

Frogs and toads range in size from tiny grass frogs that could sit comfortably on a dime to large bullfrogs that may be 25 centimeters in length. Females are usually larger than males. Frogs and toads cannot survive freezing. In winter they burrow deep into mud or soil.

A mphibian skin is kept moist in several ways. Frogs, for example, have mucus glands on the skin. These glands secrete a slimy substance that protects the skin from drying out. One difference between frogs and toads is their skin. A frog’s skin is usually smooth. A toad’s skin is somewhat rough. Toads have relatively dry skin. They avoid drying out by confining most of their activity to nighttime.

Amphibians have skeletons strong enough to support their weight. Most have four limbs, which are specialized for various functions. Frogs and toads, for example, have strong back legs for jumping on land. Amphibian’s feet are clawless. In many species, the feet are webbed for swimming.

Almost everyone has seen a frog or a toad at some time. Most people have heard the noises frogs and toads make too. These noises are mating calls. You have probably heard these calls from swamps, ponds, lakes and damp forests in the spring. The noises go on all night, from sunset to sunrise.

Digestive System

As adults, frogs and toads eat insects, spiders, and other small animals (are carnivorous). Large bullfrogs have been known to eat small fish and small ducklings. A frog uses its sticky tongue to snare insects. The frog’s tongue attaches to the front of the mouth and flips outward to catch its prey. Small teeth that line the upper jaw and two vomerine teeth that project from the roof of the mouth aid in holding prey. In the mouth, food is moistened by saliva produced in salivary glands. The food then enters the esophagus, the tube that leads to the stomach. Enzymes secreted by glands in the stomach break down food further. Food leaves the stomach through an opening called the pylorus and enters the duodenum, the first part of the small intestine. The small intestine is a long tube in which most of the absorption of food chemicals occurs. A thin, tough membrane called the mesentery holds the intestine in place. Food next passes to the second part of the small intestine, the ileum. From there it travels to the large intestine, where much of the water in the food is absorbed. Any remaining waste then enters the cloaca, a cavity that leads to the outside.

Skeletal system

A frog’s short trunk has a backbone consisting of nine vertebrae. The cervical vertebra, located at the anterior end of the backbone, allows the head to move. Seven trunk vertebrae connect the cervical vertebra to the sacral vertebra, which supports the hind limbs. Attached to the sacral vertebra is the urostyle, a long, undivided portion of the backbone that forms a ridge along the frog’s back.

Nervous system

The frog’s brain is the central organ of its nervous system and processes information from the various sense organs. A frog’s brain looks like a mere widening of its spinal cord. However it functions as a true brain. It consists of six parts: olfactory lobes, cerebrum, optic lobes, cerebellum, medulla oblongata, and spinal cord.

Hearing, Vision, and Vocalization

Amphibians rely on their senses to find food and evade predators. Amphibians lack external ears but have well-developed internal ears. Hearing is most acute in frogs, which typically have a middle ear cavity for transferring sound vibration from the eardrum, or tympanum, to the inner ear. Frogs and toads also use their keen hearing in communication with one another. Using a true voice box or larynx, and a large, expandable vocal sac attached to the throat, they produce a wide variable of vocalizations which they use in mating and territorial disputes. Salamanders, caecilians, and some frogs lack an eardrum. These amphibians cannot hear high-frequency airborne sounds, but they are able to detect vibrations that travel through the ground or water. Neither salamanders nor caecilians have a true voice box but when threatened, some salamanders can produce yelps or barking sounds.

Vision is also critical for some amphibians. Frogs have bulging eyes that protrude from either side of their head, enabling them to watch for danger and search for prey in nearly every direction. Caecilians, which live underground, are the only amphibians that are blind. Caecilians are equipped with eyes but these are covered by skin and sometimes by bone. To gather information about their surroundings, caecilians use two small tentacles—one on each side of the head—to detect chemical changes in the environment. For a sense of smell and taste, amphibians use an organ in the roof of the mouth called Jacobson’s organ. This organ probably detects chemical changes inside the mouth.

Respiratory system

A frog takes oxygen into its body in three ways. A frog breathes through its lungs, its mouth, and its skin.

To force air into its lungs, a frog lowers the bottom of its mouth while keeping its mouth shut. This procedure takes temporary vacuum in the mouth. As a result, air from the outside rushes in through the nostrils to fill the vacuum. When the frog raises its mouth and closes its nostrils, air is forced on into the lungs.

Frogs diffuse some oxygen into the blood through the membranes of the mouth, which are rich in blood vessels. This process is called mouth breathing.

Frogs also respire by absorbing dissolved oxygen directly through the skin. Most carbon dioxide also leaves the frog’s body by diffusion through the skin.

Lungs and circulation

While living on land, most amphibians breathe air just as people do. To breathe in this way, amphibians have lungs. Lungs are organs in which oxygen from the air can pass into the blood of an organism. Also, carbon dioxide – a waste product – leaves the blood, enters the air in the lungs, and is breathed out. Suppose we follow the path of blood as it travels through the body of an animal with lungs. The blood is just coming into the lungs. Call it used blood because it has given up its oxygen and has picked up carbon dioxide as it traveled through the body. In the lungs the used blood gives up carbon dioxide. The blood picks up oxygen to become what we will call fresh blood. The fresh blood is now ready to carry oxygen to all parts of the body. The blood first goes to the heart. The heart then pumps the blood to all parts of the body except the lungs. As the blood moves through the body, it «delivers» oxygen to all living cells. The blood also picks up waste carbon dioxide as it moves through the body. The blood moves back to the heart. Now the heart pumps the used blood to the lungs. The carbon dioxide is removed and a new supply of oxygen is picked up. The blood is fresh again, and the cycle starts over again.

This kind of circulation requires a heart that has either three or four chambers, or sections. Amphibians have a three-chambered heart. Some other kinds of air-breathing animals, including people, have a four-chambered heart. A three-chambered heart allows some mixing of used and fresh blood. In animals with a four-chambered heart, there is no mixing. The frog, like all amphibians, has a three-chambered heart. A membrane separates the atrium into two chambers. Blood returning from the body enters the right atrium. Blood returning from the lungs enters the left atrium. Both of these chambers contact, and blood is pumped into the ventricle. There oxygen-poor blood from the body and oxygen-rich blood from the lungs mix and is forced out of the heart. Some goes to the lungs and some goes to the rest of the body.

Reproduction and metamorphosis

Most amphibians must return to the water to reproduce. Females release great numbers of eggs into the water. Some female frogs can lay hundreds, or even thousands, of eggs. Males then release great numbers of sperm into the water, too. As with many fish, male amphibian spreads sperm directly over the eggs. The eggs and the sperm combine there. Amphibian eggs have jelly around them, but they do not have shells. Like fish eggs, amphibian eggs would dry out if they were not in water. The eggs are fertilized externally. But not all the fertilized eggs will hatch. Fish, birds, and other animals will eat many of them.

Frogs and toads go through stages as they change from an egg to an adult. As with insects, this set of changes is called metamorphosis. Other amphibians also go through metamorphosis, but the steps may not be exactly like those for frogs and toads. A frog egg hatches into a small larva called tadpole. At this time the tadpole is very small and has lidless eyes. It has a tiny tail and a sucker-like mouth. The tadpole uses its mouth to attach itself to some surface.

The tadpole soon grows some gills outside its body. Its body and tail grow longer. It begins to use its mouth for eating. Most tadpoles feed on algae at the water’s surface. As a tadpole grows, it looks somewhat like a fish with no fins except a caudal fin. Many changes are taking place inside the tadpole. The first sign of metamorphosis is the appearance of hind legs. The legs grow rapidly. Front legs appear next, and the tail becomes smaller. The tadpole cannot eat at this stage. It gets its energy by absorbing its tail. You could say the tail is being used as «food». In much the same way, a person who is dieting to lose weight is using fat from his or her body as food.

Lungs soon form inside the animal, and it begins to come to the surface for air. Before the tail is completely gone, the young adult will crawl out of the water.

Amphibians are ectothermic. Their temperature changes with their surroundings. For example, the activity level of certain kinds of frogs decreases when the temperature drops in the fall. At this time, these frogs bury themselves in the mud and hibernate. This means that the frogs stay inactive until spring. Since they are inactive, the frogs require little food. Special fat bodies inside the frogs help keep them alive until they can feed again in the spring. Some kinds of amphibians also spend a period of inactivity in very hot weather. This period called estivation usually occurs during the dry season when bodies of water tend to dry up between summer rains. Since the amphibian’s body can dry out very quickly from lack of water, some amphibians must bury themselves in the mud. During estivation, the body processes of these amphibians slow down – although not as much as they do during hibernation. Estivation usually does not last as long as hibernation.

Salamanders

There are more kinds of salamanders in the eastern half of the USA than there are anywhere else in the world. Salamanders may be from five centimeters long to one and a half meters long. However, most salamanders are less than 12 centimeters long.

Salamanders are not seen very often. They do not make any noise and they stay hidden during the day. About the only way you might see one in nature is if you happen to turn over a rock or pieces of wood under which the salamanders are hiding.

Salamanders look very different from frogs and toads. Salamanders have a tail. Because they have a tail, they are sometimes thought to be lizards. But lizards are not amphibians. They are reptiles. Lizards have scales, claws and five toes on each foot. Salamanders have no scales or claws and they have only four toes on each front foot.

Salamanders eat insects, worms, and other small animals. Some of them also eat algae or animal’s refuse.

Salamander’s eggs are fertilized internally. The female uses her cloaca to pick up a packet of sperm, or spermatophore, that the male has deposited on a leaf or stick. Aquatic species deposit the fertilized eggs in the water, and land species place them on the ground. All newly hatched young salamanders have gills but most develop lungs as adult.

5 .9.4. Class Reptilia

General characteristics

Alligators, crocodiles, turtles, lizards, and snakes are reptiles. A reptile’s skin is covered with scales or with tough, horny plates that keep the reptile from drying out. Since a reptile loses little water through its skin, some reptiles are able to live in very dry places. Some even live in the desert.

Though reptiles no longer rule the land, they still play the major ecological role in most of the world’s biological communities. This is especially true in the tropics, deserts and warm grasslands.

Modern reptiles have a waterproof skin and produce amniotic eggs. All reptiles also have strong, bony skeletons and well-developed lungs. Most of them have two pairs of limbs of amphibians. Reptilian feet have toes with claws that are used for digging or for climbing on trees and rocks.

The nervous system of reptiles is similar to that of amphibians. The brain is small in relation to the body. Despite the small brain, reptiles have shown complex behavioral patterns, including elaborate courtship rituals.

The excretory system of reptiles is modified to minimize water loss. Water is absorbed into the body through the intestine, bladder, a pair of kidneys, and cloaca.

Like amphibians, most reptiles have a three-chambered heart. However, crocodiles and their relatives have four-chambered heart, a structure more efficient at separating oxygenated and deoxygenated blood.

Like many other kinds of animals, reptiles lay egg, which is called amniotic egg. Because most reptiles live on land, their eggs and sperm are not left in water as those of most amphibians are. Instead, the male deposits sperm inside the female’s body. There, the sperm combine with eggs. A protective covering, or shell, forms around the eggs. Then, in time, the eggs are laid in some kind of nest.

The shell is important to the baby reptile that is developing inside. The shell keeps the reptile from drying out. It also protects the reptile from other animals during the time before hatching. Another part of the egg – the yolk – is also important to the baby reptile. The yolk is the food for the reptile before the egg hatches.

When a reptile egg hatches, the baby reptile looks like a tiny adult. That is, a reptile does not go through metamorphosis as it becomes an adult.

Class Reptilia is divided into several orders, and among them (the most famous) are:

Crocodilia (the crocodiles, alligators, and related species adapted for life in shallow water).

Chelonia (turtles and tortoises). Their bodies are enclosed within two bony shells.

Squamata (lizards and snakes, reptiles with long slender bodies).

The order Crocodilia includes about 250 species of alligators, crocodiles, and their smaller relatives, caimans and gavials. Crocodilians are active water-dwelling carnivores.

A lligators and crocodiles have large jaws and a powerful tail. Like most reptiles, they eat other animals. Small animals may even be swallowed whole. The crocodile group is made up of reptiles that live in water and are good swimmers. The crocodilian’s eyes and nostrils rise above the rest of its head. This adaptation allows a crocodilian to remain submerged and still see and breathe. A special valve between its mouth and nose passage keep water from entering the breathing passage even when the animal opens its mouth to feed under water. Crocodilians differ from one another mainly in the structure of their head and teeth. The alligator has a broad head with a rounded snout. The crocodile has a triangular head with a pointed snout. In alligators, the teeth of the lower jaw fit inside the teeth of the upper jaw. In crocodiles, most of the upper and lower teeth mesh together. Many of these reptiles, such as crocodiles drift quietly in the water with their nostrils and eyes sticking out. In this way, they hunt their prey which includes fish, swimming birds and turtles. Large alligators and crocodiles can be quite dangerous to people and to other large animals. The alligator or crocodile usually attacks other animals in the water. The alligator or crocodile will pull the other animal under the water and hold it under until the animal drowns. While holding the other animal under, the alligator or crocodile keeps its own nostrils above the water.

The female lays eggs in piles of rotting plant material or in holes in the ground. She stays near the nest until the eggs hatch. Then she stays with young animals until they can take care of themselves. Other kinds of reptiles do not stay with their young in this way.

Alligators and crocodiles often make dens in riverbanks. The entrance to the den is underwater but the «living room» of the den is above the water level.

T urtles and tortoises make up the second group of reptiles. These reptiles live in armour-like shells that provide protection. Some turtles, such as a box turtle, have a shell that covers their whole body. The shell of some other kinds of turtles covers only part of their body. They also have strong, sharp beaks without teeth. The limbs of sea turtles are flippers. The land turtles – tortoises – are especially well-known for their long life spans. Some species have lived for over 150 years in captivity.

Most turtles move rather easily in water. However, most turtles cannot move very fast on land. Their shell is often just too large and heavy.

You may remember that amphibians go to the water or to a place that is wet to lay eggs. But turtles that live in the sea do just the opposite. They return to the land to lay eggs. One such turtle is the large green turtle. Its legs are more like paddles than legs, so it moves very slowly on land. Its eggs are laid in beach sands above the usual high-tide levels.

A female turtle leaves her eggs to hatch in the warm sand. After the eggs hatch, the baby turtles must make their way down to the sea by themselves. The shell of a very young turtle is not hard enough to protect the turtle. Many of them are caught and eaten by birds and other animals before they get to the sea.

Members of the order Squamate are the most successful of the modern reptiles. The two major kinds of squamates are lizards and snakes.

L izards. Most of these reptiles have four legs with five clawed toes on each foot. Lizards range in size from five centimeters to about three meters. Monitor lizards, such as Komodo dragons, are the largest lizards. They can weigh 140 kilograms.

Lizards live in brush piles, near fallen trees, in rocky places, in burrows, or in any other place where they can hide from their enemies. Lizards have a keen sense of sight and smell to help them find food. They eat insects, spiders, snails, worms, mice, young birds, and even other lizards. They have teeth for grabbing and holding the animals they eat, but they do not use their teeth for chewing. Instead, they swallow their food whole. Some animals that eat lizards are other reptiles, foxes, coyotes, and birds.

Lizards are protected by their alertness and their speed in seeking cover. They may also protect themselves by fooling their enemies. Some lizards have a tail that breaks off easily. An animal trying to catch a lizard would be most likely to grab the tail. If the tail breaks off, the lizard might be able to escape. Then the lizard will grow a new tail.

Snakes make up the fourth main group of reptiles. Snakes are like lizards in some ways. They tend to live in the same kinds of places and eat the same kinds of food. But there are many differences between snakes and lizards. Snakes do not have legs as lizards do. Snakes have jaws that open very wide and have a single row of scales along their belly. Lizards do not. Snakes and lizards occasionally shed their outer skin coverings. Snakes are also unusual in that they do not have limbs or ear openings.

All snakes molt. That is, they lose their outer layer of skin. Even the coverings of their eyes come off. The skin first becomes loose near the snake’s head. After the snake gets its head out of the old skin, the snake crawls out of the rest of the skin. Sometimes the skin turns inside out as the snake crawls out. A snake may molt more than three times a year. Generally, young snakes molt more often than old snakes because young snakes grow more rapidly.

Some snakes are poisonous. That is, they put poison into whatever they bite. However, most snakes are not poisonous. Of about 2,700 kinds of snakes in the world, only about 270 kinds are poisonous. Snakes are helpful to people because they eat harmful animals such as rats, mice, and certain insects.

Snakes can swallow animals larger than themselves. Their lower jaw is not attached to the upper jaw in the same way that it is in other reptiles. This allows a snake to open its jaws very wide. Snakes have unusual jaws that enable them to swallow prey much larger than the diameter of their own heads.

Poisonous snakes have two long, hollow teeth called fangs. Fangs are like hollow needles through which poison flows into a «bite». Most poisonous snakes in the USA are pit vipers. They are called pit vipers because of a depression, or pit, that they have between each eye and the nostrils. The fangs of these snakes are usually folded back against the roof of their mouth. Only when the snake wants to bite are the fangs out. Rattlesnakes, copperheads, and cottonmouths are pit vipers.

Another kind of poisonous snake is the coral snake. Its fangs are not folded back against the roof of the mouth. The fangs stay in one position. A coral snake has bright bands of yellow, red and black around its body.

Snakes do not like to be around people. All snakes avoid people if at all possible. However, snakes will strike when they are cornered, stepped on or surprised.

Throughout the world deaths from snakebites may number as many as 30,000 a year. The number of deaths from snakebites is greatest in India and countries in South-East Asia.

Most life scientists believe that, at one time, there were much more reptile groups. Fossil remains suggest that the huge dinosaurs that once roamed the Earth were reptiles. Other reptile groups probably included ocean-dwelling and flying species. Scientists are still not completely sure what caused these organisms to die out.

5.9.5. Class Aves: Birds

Birds are warm–blooded animals with complete double circulation and a skin covered with feathers. The forelimbs are wings. Flight is not a feature that makes birds unique, however. Birds share this ability with bats and insects. Birds, like the reptiles, fertilize their eggs internally and lay eggs with shells that do not dry out on land. Their embryo is enclosed in an eggshell with protective membrane. About 8.590 species are known. In their skeleton structure birds are essentially reptiles highly specialized for flight. They have feathers, however, one of the distinctions of this class, and unlike the reptiles, they maintain a high and constant body temperature, which may in part permit the high-energy output required for flight. Their bodies are lightened by air sacs and also by hollow bones.

The most massive bone in the bird skeleton is the breastbone, or sternum, which provides the keel for the attachment of the huge muscles that operate the wings. Flying birds have jettisoned all extra weight, the female reproductive system has been trimmed down to a single ovary, and this becomes large enough to be functional only in the mating season. The pectoralis muscle lowers the wing and the supracoracoideus raises it. A group of vertebrae, called sacral vertebrae, are located at the lower end of the spine. The femur is the upper leg bone, the lower leg bone is the tibia.

Most of the common nonflying birds such as the penguin and the ostrich are believed to have evolved secondary from flying types.

Characteristics of birds:

• The body is divided into neck, head, trunk and tail;

• Bones are lightweight and filled with air;

• Air sacs are found throughout the body;

• Birds lack teeth; jaws are covered by horny beak;

• Front limbs are modified into wings; hind limbs are adapted for perching, hopping, swimming or other similar activity;

• Body temperature remains constant;

• The heart has four chambers;

• Reproduction involves the production of an amniotic egg most species incubate in a nest.

One big difference between birds and all the other animals you have studies so far is that birds are warm-blooded. The body temperature of cold-blooded animals is about the same as their surroundings. So, cold-blooded animals can not live where the air temperature stays below the freezing point of water for long periods of time. The body temperature of the warm-blooded animals does not depend on their surroundings. Their body temperature always stays about the same. A temperature is about 41 degrees of C is normal for birds. Birds keep this high temperature because they have a high rate of metabolism. That is they turn food into energy rapidly. But birds also need a lot of energy because they are active. They also need a lot of energy to keep their body temperature up – especially in cold weather. Because in their high rate of metabolism birds eat a very large amount of food for their size. In fact, some birds eat their own weight in food each day.

The ways birds are different and similar

Birds differ from other vertebrates in that they have feathers. There are important differences among birds. Birds are classified on the basis of a number of characteristics, including body structure and behavior, into 4 common types: flightless birds, water birds, perching birds, birds of prey. Each group of birds is adapted to a specific way of life. Legs, feet, and beaks differ according to the ecological role of the group. Perching birds that live in trees have curved toes that are able to cling to branches. Predators capture their prey with sharp claws called talons. The foot of a duck is suited for paddling and swimming. The foot of the hawk is suited for grasping prey. The beaks of birds also vary widely in their use and structure. The thick beaks of some finches are suited for cracking seeds. An eagle’s beak is adapted to cutting and tearing flesh. A woodpecker’s beak is used for drilling through wood.

Many birds are especially helpful for humans because they eat the insects and rodents that damage food crops. People often enjoy watching or photographing different species of birds. Nest-building chores are of particular interest to many bird-watchers. Birds can also be harmful to human. Some birds carry diseases, such as influenza. Many species of birds eat our valuable crops. Some birds, such as pigeons, foul buildings and statues. Pigeons have also been known to peak away the material between bricks. This material contains lime that the pigeons use in their eggs.

Flight

Most birds fly. The structure of birds is well suited for flight. Their skeleton, for example, is light in weight. Yet it is very strong. Their bones are hollow and have thin walls. Flying requires a very good circulatory system to get large amounts of food and oxygen to all the cells of the body.

The wings are very important in flight. The part of the bird’s wing that is closest to its body is shaped like an airplane wing. This shape - called airfoil – gives each wing an upward push, which the bird needs to stay in the air. The upward push – called lift – is made possible because air moves faster over the wing than under the wing. The wings not only provide lift but also provide thrust, the force needed to move the bird forward. A bird has large, powerful breast muscles for moving its wings. In most birds, these powerful muscles are able to work steadily for long periods of time.

Some birds, such as swallows and gulls, have wings that are best for continuous flight. Other birds, such as quail and pheasant, have wings that are best for short, rapid flight. Still other birds, such as ostriches, are flightless. They cannot use their wings for flight at all.

A bird’s feathers are its plumage. Different species of birds can be recognized by the colors and designs of their plumage. Males of a species are often more brightly colored than females, as shown by the pheasant pair. The duller color of the female can help her blend in with her surroundings when she hatches eggs and cares for her young.

Feathers are outgrowths of the bird’s skin that facilitate flight and also provide insulation. Feathers evolved from the scales of birds’ reptile ancestors. Feathers grow from small sacs in the skin called follicles. They first appear as small, dark pin feathers that have a scaly covering. Mature feathers are strong and light because of their unique structure. Feathers have a central cylinder or rachis running their length. The part of the rachis that extends out of the skin is solid, while that beneath the skin is hollow. This hollow section is known as the quill. Birds have 4 types of feathers: contour, down, filoplumes, and bristles. Contour feathers are smooth, sleek feathers that cover the head, body and wings. These feathers protect and streamline the bird. Down feathers are soft, fluffy feathers found under contour feathers. Down feathers insulate birds. Filoplumes are short, thin feathers that look like hair. Bristles, also short, hair-like feathers, are located near the bird’s nostrils. Bristles filter dust.

Digestive system. Birds must eat large quantities of food to generate the energy needed for both flight and temperature regulation. Different species of birds have different diets. The digestive system of a bird processes a large amount of food at a time. Food is taken in through the mouth, travels down the esophagus, and enters a large crop, which is used primarily for food storage. Upon leaving the crop, food moves into a two-chambered stomach. The first chamber is the thick-walled proventriculus where food is mixed with digestive enzymes. The second chamber of the stomach is the gizzard. The gizzard functions as a substitute for teeth; it grinds up the food. A bird’s gizzard has a hard, horny lining. The duodenum, the coiled intestine and the other digestive organs of the bird are similar to those in other vertebrates

Respiratory system

A bird’s high body temperature and level of activity require a large amount of oxygen. Air is drawn into the body through the mouth or the two nostrils in the beak. The air then passes through the trachea into a pair of tubes called bronchi. Each bronchus leads to one of the lungs. Much of the inhaled air travels into air sacs – cavities that make the bird lighter when filled with air. Air sacs extend into the body cavity and even into the toes. Because of the air sacs, a bird’s respiratory system may take as much as 20 percent of the body volume. Some of the air in the body is used for making sounds. Birds do not have vocal cords. They produce their calls instead by means of their syrinx, or “song box”. The syrinx is located at the bottom of trachea. It contains a pair of vibrating membranes. Muscles attached to the membrane change the pitch of the sound.

Circulatory system

Birds have four-chambered hearts. The right and left sides of the heart are completely separate. Each side of the heart is divided into two chambers, an upper atrium and a lower ventricle. The right side of the heart receives oxygen-poor blood from all parts of the body and pumps it to the lungs. After the blood is oxygenated in the lungs, it travels to the left side of the heart. From there it is pumped out to the body through the aortic arch. A bird’s heart beats very rapidly, although the rate varies greatly from species to species. A mourning dove’s heart rate ranges between 135 and 570 beats a minute. Thus, the blood circulates rapidly. Rapid circulation is important in very active animals like birds.

Excretory system

The excretory system of a bird is modified to converse water. Almost all the water is removed from urinary products as they pass from kidneys to the cloaca. The primary form of nitrogenous wastes is a white solid compound called uric acid. Wastes from the intestine and from the kidneys are excreted together.

Nervous system

The nervous system of a bird is highly developed. The brain has three well-developed areas: the cerebellum (controls flying and walking by coordinating movement), the midbrain tectum (the center of vision), the optic lobes (the place where visual impulses are interpreted is located in this area). Birds have keen senses of hearing and balance but poor senses of smell and taste. They depend more on their vision than on any other sense. Birds have extremely large eyes that detect slight movements. Although the eyes are unable to rotate, a bird still has a wide range of vision because its neck is flexible due to a large number of vertebrae.

Reproductive system. Male birds produce sperm in testes. The mature sperm pass from the testes into a long tube, called the vas deferens. The lower part of the vas deferens is enlarged to form a seminal vesicle where the sperm are stored until mating occurs. The female of most bird species has only a single ovary, located on the left side. Eggs produced in the ovary are released into the body cavity. After mating occurs and sperm fertilize the eggs, the eggs enter the oviduct. The oviduct is a long tube that leads to the cloaca. As eggs travel through the oviduct they are covered by albumin, a nutritious protein that makes up the egg white. Later, the eggs are covered by shells made primarily of calcium carbonate, also called lime. Eggs with shells leave the body through the cloaca. Eggs that are not fertilized also receive albumin and a shell.

Migration. Many bird species mate, build nests, and raise their young in one part of the world and then move to another to avoid winter. A common sign of winter’s approach is the sight of birds overhead, flying south. Similarly, the return of birds to the north signals the coming of spring. This regular, seasonal movement is known as migration. These trips are often long and difficult. Year after year birds migrate over the same paths, called migration routes. Birds find their way partly by instinct and partly by learned behavior.

5.9.6. Class Mammalia.Mammals

Characteristics of Mammals