Amphibians are members of the class Amphibia, subphylum Vertebrata, phylum Chordata. The class Amphibia includes frogs, toads, salamanders, newts and caecilians. Amphibians are characterized by a glandular skin without external scales, by gills during development (and in adulthood in some), and by eggs that may have jelly coats but develop without formation of extraembryonic membranes such as the amnion. Most amphibians also have four limbs. Limbs and lungs are adaptations for life on land; the limbs evolved from the ancestral fishes' lobed fins. The scales and amniote egg evolved by reptiles are further adaptations for life on land and distinguish reptiles from amphibians.
They are tetrapods meaning that they have four limbs. This facilitates them to move on land. The limbs evolved from pectoral and pelvic fins. Their skin is soft, thin and glandular and magid. They lack scales except for caecilians. Caecilians have scales similar to those of fish. They are ectothermic. They breath with both lungs and gills. Gills are used during larval stages which are replaced by lungs in adult stages. Cutaneous respiration is also in many amphibians.
Life Cycle of Frog
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Masses of fertilised eggs hatch revealing limbless, gill bearing tadpole larvae. They feed and grow and undergo transformation. Hind legs appear and gradually lengthen. The tail shortens, larval teeth are lost, gills are replaced by lungs, eyelids develop and forelegs emerge. In short time of few weeks the metamorphosis is complete resulting into an adult frog.
Terrestrial and Aquatic Habitats
Mostly animals are in water. They need many structural and functional characteristics which are required to survive on land. They are present in amphibians.
- Differences in water content : In aquatic environment water is always present. In terrestrial environments water is extremely in variable amounts. Less water overall.
- Greater oxygen content of air : Aquatic animals spend more energy taking oxygen from the water that the land animals do from air.
- Water is more dense: water provides more buoyancy and provides support for locomotion even though it is harder to move in water than in air. Where as survival on land requires a skeleton and a good strong one for larger animals.
- Constancy of water temperature: water has high capacity. It does not change temperature unless large amounts of heat are added or removed. Most aquatic animals, especially marine species. Do not have problems with drastic temperature changes that occur on land.
- Higher variety of habitats: on land as opposed to aquatic environments.
- Greater opportunities for breeding on land: safe shelter is available for eggs on land then in water.
Origin and relationships of amphibians
For land animals it is a must that they support their own weight, resist drying, cope with temperature changes, and extract oxygen from air. To survive on land it is necessary to have lungs for respiration. Gills with their fine filaments and gill lamellae require water to prevent collapse and desiccation along with the lungs a more efficient circulatory system is required. All fishes have a two chambered heart which pumps blood through gills to get oxygen and from there to the remainder of the body before returning to the heart. This system is sufficient for aquatic beings who are surrounded by water from which can diffuse into some parts of the body. Aquatic animals are less active and so do not require a highly efficient circulatory system.
On land however thick skins are important to prevent desiccation but restrict diffusion of oxygen from the air. Also, many terrestrial animals have a very high metabolic activity. Therefore all land animals have a double circulatory system which allows oxygenated blood to be returned directly back to the heart before being pumped to the body.
Components of the double circulatory system
- Pulmonary circulation: the is blood is transported to the lungs first where the blood is oxygenated and provides the lungs with oxygen too. Then the blood is transported back to the heart from where it is transported to the rest of the body
- systemic circulation : supplies blood with oxygen to all of the body except the lungs.
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The fossil records indicate that amphibians appeared abruptly in the record as fully developed amphibians with all of the characteristics aforementioned present and they all have very well developed lungs, three chambered heart for double circulation, well developed limbs, well developed skeletal system, well developed muscle system and a very well developed CNS. No transitional forms found in fossil record that would indicate any gradual appearance of these necessary, terrestrial vertebrate characteristics.
Structure and History
- Caecilians: Many species are blind as adults. These are called caecilians. Characterised by long, slender body covered with small scales in some species. They have no limbs and no postanal tail.As they are burrowers their eyes are replaced by specialised sensory tentacles on the snout. They are hardly seen due to their burrowing nature.
- Salamanders, order Caudata( tailed amphibians): newts are also included in this order. Salamander are the least specialised form of all amphibians. Some species are aquatic and never produce true lungs, their gills are the major respiratory organ for their entire life cycle. Rest of the amphibian characteristics are present. They exist for short periods out of water. Most salamanders are carnivorous preying on worms, small arthopods, and small molluscs. Most eat only things that are moving. Their food is rich in proteins and so they produce a lot of nitrogenous waste, usually urea which less toxic and requires less water for excretion. They use the carbon skeletons of the protein derived amino acids for their catabolic (energy) and anabolic (biosynthetic needs). Therefore they do not store much fats or glycogen. Salamanders are ectotherms. They derive heat from the surroundings. Thus have a low metabolic rate.
Terrestrial salamanders live in moist places under stones and rotten logs, usually not far from water. They do not show much diversity in regard to breeding habitats, due some what restricted habitats in which they are found. Their eggs are fertilised internally. The females picks of the packet of sperm that has been deposited by the male on a stick or a leaf. Aquatic species lay their fertilised eggs in small group like clusters under logs or in holes of soft dirt. Many species remain near their eggs to guard them. The off springs resemble their parents. The larvae undergo metamorphosis during development, but not to the extent that tadpoles of frogs or toads do.
All salamanders hatch with gills, but during development, they are lost in all except aquatic forms or in some species which do not undergo complete metamorphosis. Terrestrial salamanders have well developed fully functional lungs. Some salamanders just respire through their skins as they not have lungs or gills. The skin contains an extensive vascular network of capillaries which allow for extensive gaseous exchange to take place just beneath the epidermis. This is called cutaneous respiration. The buccal cavity has a highly vascularised membrane system that supplements the cutaneous respiration in lung less and gill less salamanders.
Frogs and toads : these are very specialised order of the amphibians. Very popular for educational purposes, but are actually poor representatives of the vertebrates body plan. That is because they lack a visible neck, the caudal vertebra are fused into a urostyle (coccyx, tailbone) which is found in advance vertebrates like mammals and their ribs are absent ( in some species) while in others it is reduced ; no true rib cage in most species.
Classification and taxonomy
Frogs and toads are divided into 21 families. The most well known north American families are:-
- Hylidae(tree frog)
- Bufonidae ( toad)
Differences between Frogs and Toads
- Smooth moist skin
- no warts on body
- narrower body and waist
- live near or in water
- longer hind legs for hopping webbed hind feet for swimming
- eggs are in massive lump of cloudy jelly
- rough drier skin
- have warts on their body
- broader, flatter body
- live on land, breed in water only
- shorter hind legs, walk rather then hop
- eggs are long and stringy, wrapped around plants.
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Amphibians, especially anurans, are economically useful in reducing the number of insects that destroy crops or transmit diseases. Frogs are exploited as food, both for local consumption and commercially for export, with thousands of tons of frog legs harvested annually. The skin secretions of various tropical anurans are known to have hallucinogenic effects and effects on the central nervous and respiratory systems in humans. Some secretions have been found to contain magainin, a substance that provides a natural antibiotic effect. Other skin secretions, especially toxins, have potential use as anesthetics and painkillers. Biochemists are currently investigating these substances for medicinal use.