Living things are classified into five kingdoms including animal, plant, fungi, protista and monera. In kingdom animalia, all members are multicellular organisms, however they do not express the similar organisational pattern. Animals can be categorised into five different groups including mammals, fish, amphibians, birds, and reptiles. They all depend on the environment such as air, food, water, and shelter to survive. Amphibians are the only vertebrates that spend a part of their life in water and a part on land. Therefore, they are different from other groups of animals. Lobe-finned fish evolved into the first amphibian about millions of years ago.
An amphibian is a small vertebrate organism that requires water or a moist environment to exist. They can survive in both terrestrial and aquatic environments and the body temperature of amphibians is dependent on the temperature of the environment, therefore, they are also called cold-blooded vertebrates. They include frogs, toads, newt, caecilians, and salamanders.
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Body
The body of an amphibian is divided into the head, trunk, and tail (salamanders), some amphibians have only a head and tail (frog). The neck may be present or absent.
They have two pairs of pentadactyl limbs, however, some amphibians have no limbs.
The skin is soft, moist, without scales, and rich in mucous glands.
Sense organs
Amphibians have a pair of olfactory lobes that are concerned with a sense of odour.
They have well developed eyes.
They do not have an external ear, however, the tympanum covers the middle ear.
Digestive system
Amphibians have digestive tracts including the mouth, oesophagus, stomach and intestine closing in a division called the cloaca.
Cloaca is participates in the digestive, excretory, and reproductive systems.
Circulatory system
Amphibians have a closed circulatory system and the heart is three-chambered, with two auricles and one ventricle.
There are two circulatory paths; one is for the oxygenation of the blood through the lungs and skin and another route is to carry oxygen to the remaining parts of the body.
However, the oxygenated blood received in the left atrium and deoxygenated blood received in the right atrium gets minimal mixing, therefore, there is incomplete double circulation.
Diet
Amphibians eat invertebrates such as blood worms, mealworms, earthworms, snails, slugs, locusts, etc, and large amphibians can eat small mammals.
Adult frogs need food only every 2-3 days and young frogs need most of the days in a week.
Excretory system
Amphibians have pronephric kidneys at the larval stage and adult amphibians are similar to aquatic vertebrates, they have the mesonephric kidneys.
They excrete the major metabolic waste as ammonia in tail form and urea in the tailless forms which is when they are on land.
These wastes are carried out by kidneys where it is separated and excreted, therefore the major excretory organ of amphibians are kidneys.
Reproduction
Amphibians involve internal (Salamander) or external fertilisation (most amphibians).
They can attract mates by producing sound, for example, in frogs, the loud croaking may be the signal for mates.
They need fresh water to lay their eggs because the eggs do not have shells and it becomes dry when they are kept on land, therefore, they must lay their eggs in water.
Metamorphosis
The changes that occur in the life cycle of an animal is called metamorphosis. In frogs, an egg hatches to release a tadpole that first develop back legs, then front legs and becomes an adult frog.
Amphibians can be classified into three groups based on orders. They have different sizes and structures, and their evolutionary history is different from one to another group.
Apoda (Gymnophiona or Caecilia)
The body is elongated and divisible into the head and trunk.
The limbs are absent, therefore they look like earthworms.
The small dermal scales are present.
They are called blind worms because their eyes are enclosed by bone or skin.
They have short or absence of tails.
They engage in internal fertilisation.
Example− Caecilians.
Urodela (Caudata)
The body is elongated and has a head, neck, tail, and four limbs at a similar length.
They have soft and moist skin.
They can breathe through the skin.
They have teeth in their jaws at both larvae and adult stages.
They are unable to produce sound.
They involve internal or external fertilisation.
Examples− Salamanders and newts.
Anura (Salientia)
The body is divisible into the head and trunk, however the head and trunk are fused.
They have four limbs and these are specialised for jumping.
They have a large mouth.
The tail is present at the larval stage and becomes tailless at an adult stage.
The skin secretions of anurans give a natural antibiotic effect because of the presence of a magainin substance.
The external fertilisation takes place in anurans.
Examples− Frogs and toads.
| Domain | Eukaryote |
| Kingdom | Animalia |
| Phylum | Chordata |
| Subphylum | Vertebrata |
| Class | Amphibia |
| Order | Urodela |
| Apoda | |
| Anura |
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Skin is a thin, permeable organ, rich in blood vessels and most amphibians can breathe through the skin. Some aquatic organisms such as frogs have gills, during the the larval stage that absorb oxygen from the water and release carbon dioxide as waste.
Amphibians are cold-blooded vertebrates evolved from lobe-finned fish. They can survive in aquatic environments at the larval stage and spend their adulthood in the terrestrial ecosystem. They are classified into the orders of Apoda, Urodela and Anura that include frogs, toads, salamanders, newts, and caecilians. They are different in the body type from one to another group, and they can breathe through their skin (gills) at a young stage and when they reach adulthood, lungs are developed, therefore they can breathe both skin and lungs.
Q1. What is the evolutionary significance of Amphibians?
Ans. Amphibians are organisms which can survive in both land and water, it is considered that they are intermediate between the aquatic and terrestrial organisms. Thus, played an important role in the evolution of terrestrial organisms.
Q2. What are some common examples of amphibians other than frog?
Ans. Salamander,axolotl larva, toads are common examples of amphibians.
Q3. What is incomplete double circulation?
Ans. Organisms like amphibians possess three chambered hearts. Such hearts have two atria and one ventricle. In these organisms, one circulatory route plays a role in getting the blood oxygenated while another route distributes the oxygenated blood to the body. But here, the oxygenated and deoxygenated blood cannot be kept completely separate but their mixing can be prevented to a great extent. Thus, it termed as incomplete double circulation.
Q4. What is the role of olfactory lobes in amphibians?
Ans. The olfactory lobe has the ability to smell, many aquatic organisms target food by odour, therefore amphibians can also find food by smell or odour.
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