The word Amphibia is the combination of two words. The word ‘Amphi’ means dual and ‘bios’ means life. Therefore, the word ‘Amphibia’ represents a class of living creatures who can survive in both land and water environment. In medical studies, amphibia represent the class of all living organisms that belong to the group ‘Lissamphibia’ and the species in the group include frogs, toads, newts and salamanders.
All of the species in the group can breathe and absorb water through their very thin skin. These organisms start their life as larvae that can survive in water and breathe with gills. Once they are fully developed, they can air-breathe through the lungs. This developmental phase of these species is metamorphosis. In some species of this group, they have skin as a secondary respiratory system but others like frogs and salamanders do not have lungs and they are dependent on their skin for breathing.
Evolution History of Amphibia
The evolution of amphibians starts from the Devonian period means around 370 million years ago. They were developed from lobe-finned (multi-jointed leg-like fins) fish that could crawl along the sea bottom. These fish had developed lungs to breathe air and their fins were developed strong that they could hoist the fish out from water if the oxygen level becomes too low to survive. After a certain period, their fins would evolve into limbs. Even with these developed fins, fish spent most of their time in water and even after the development of lungs they used to breathe through gills only.
During the Devonian period, they developed nostrils and more efficient lungs, neck, sturdy limbs to help walk on the land and a tail with fins that would help to swim in water as well. They also learnt to stay out of water for longer periods. Lungs became strong and skeletons became heavier to support their weight on the land. Skin also developed to the next level to retain body fluids and became resistant to desiccation. Fins became smaller in size developed amphibian ears for hearing on dry land. At the end of this era, they were so developed that they could crawl and propel themselves with forelimbs like an elephant seal.
In the early Carboniferous period (from 360 to 345 million years) they were the food for the carnivorous amphibians. In this period also, they used to go back to the water to lay their shell-less eggs. Even today also, their life starts from larvae with gills like their fish ancestors.
During the Triassic period (from 250 to 200 million years), they were suppressed to their sizes and their importance to the outer biosphere by the reptiles. The modern amphibians’ group, Lissamphibia, survived and may have branched off from the extinct groups during two periods, the late Carboniferous period and early Triassic period.
Skin, Skeletons, Respiratory and Reproduction System of Amphibia
Amphibians have skins that are permeable to water and respiration through their skin allow them to keep themselves deep down in the pond. Their skin has mucous glands which release secretion to keep them moist and these glands are at their heads, backs and tails. In addition to these glands, amphibians have other glands that release poisonous and distasteful secretions. Some of these secretions are very poisonous and can be lethal to humans. These species can change their skin colour. An amphibian is toxic or not can be determined by the vividness of its skin colour; the more vivid the skin colour more the creature is poisonous.
Like other tetrapods, amphibians have a homologous skeletal system. They have hollow and lightweight bones that are connected in a strong skeletal system so that they can support the head and body. In most amphibians, like frogs, their hind legs are longer as compared to the forelegs as this helps them to jump or swim quickly. As per the adaptations of the way of life, the feet of amphibians are short or long. If the amphibians are walkers or runners their hind legs are so long as compared to frogs. Webbing between the toes is also another kind of modification that helps them to swim faster. Another modification is the broad adhesive toes for climbing.
The development of lungs in the amphibians is in the primitive stage and is not developed fully. Most amphibians use their skin for exchanging gases from water or air and this type of respiration is cutaneous respiration. The skin of these organisms is maintained moistly by releasing secretions from glands located on their head, body and tail. This helps to diffuse oxygen into the skin at a high rate. Frogs have both the respiratory systems as they live on land as well as water but some of the species rely solely on the cutaneous respiration system. In the larval stage (tadpoles) of frogs, they have gills for respiration.
Reproduction in amphibians happens only in water as their eggs are shell-less and to survive they need moisture. Some amphibia need freshwater for reproduction whereas some lay their eggs on the land but making the surroundings moist by all means. Some species of frogs do not require water as their eggs hatch directly into the miniature versions of adults and become tadpoles within the egg. The successful reproduction of amphibia depends not only on the rainfall but also the seasonal timings. In the rain forests, the amphibia breed throughout the year but in tropic areas, the breeding depends on rainfall, hot and moist season or in the spring. In caecilians and salamanders, fertilization happens inside the female partner. On the other hand, frogs use external fertilization.
FAQs on Amphibia
Q.1: How the blood circulation does happen in amphibia?
Answer: Two types of blood circulation happen in amphibia and this circulation depends on the stage of development. These species have two stages, namely, juvenile or tadpole stage and adult stage. In the first, i.e. juvenile stage, blood circulation happens like in the fish. In a single loop, the heart pumps the blood which flows through the gills where it is oxygenated and spread around the body and moves back to the heart. However, in the adult stage, when lungs develop and gills disappear, the contraction of the ventricle pumps deoxygenated blood through the pulmonary artery to the lungs and pumps oxygenated blood throughout the body.
Q.2: How does the nervous system in amphibia similar to fish?
Answer: In amphibia, the nervous system works in the same way as in fish. Like other vertebrates, amphibia have a central brain, the spinal cord and nerves throughout the body but the brain is less developed as compared to reptiles, birds or mammals. The brain, same as other species, also has the cerebrum, midbrain and cerebellum. Their functions are similar to the function in other species.
Q.3: Explain the fertilization process in salamanders.
Answer: In salamanders, fertilization takes place internally. In this, the male releases a spermatophore packet (a gelatinous cone-shaped) on the land or water. This packet is then grasped by the female through the lips of the cloaca and takes it to the vent. This packet will be there until ovulation that may happen after many months. In some cases, the spermatophore is placed directly into the cloaca of the female or is guided to it by the male or by restraining the female with an embrace called amplexus. Also, in certain primitive salamanders, external fertilization happens. In this process, first, the female laid the eggs in water and the male releases the sperms on them. This process is similar to frogs.