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. Anurameans “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.
Amphibian skin is kept moist in several ways. Frogs, for example, have mucus glandson 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.
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.
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.
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.
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 estivationusually 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.
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
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