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Advances in the Study of Behavior. This is true primarily of some forms of bacteria, and indeed scientists believe that the first organisms to appear on Earth's surface were anaerobic. They all have four limbs except for the legless caecilians and a few species of salamander with reduced or no limbs. The respiratory system View More. Its eggs are laid on the forest floor and when they hatch, the tadpoles are carried one by one on the back of an adult to a suitable water-filled crevice such as the axil of a leaf or the rosette of a bromeliad.
Biology Dictionary © Macroevolution.net
Mammals belonging to the Order Chiroptera bats exhibit special adaptive features that enable them of true flight:. Many species such as the Crescent nail-tail wallaby, Steller's sea cow and Flores cave rat have even faced complete extinction. The overall Mammal population throughout the world is still deteriorating with around one-quarter of the total species being threatened with extinction.
The wild life conservation units in different countries are taking important measures for protecting different species from the class Mammalia. Various other species are also protected by law in different countries. The Chinese Water Deer also known as the Asian water deer is a small ungulate that is known for its long fang-like canine teeth and belongs to the w.
The Pygmy Hippopotamus is a species of dwarf or very small hippopotamid, compared to the other common hippo species. By size, these mammals are about. The Bat-eared Fox is a species of very ancient canines that was widely distributed in the middle Pleistocene era, around , years ago. The Himalayan Tahr, also known as kaarth, meshi, and taheer, is a species of grazing ungulates that are found in parts of the Himalayan mountain range.
The Indian wolf is a grey wolf subspecies found in west and South Asia. It was initially given species status as Canis pallipes but was assigned as a. Javan rhinoceroses are rhinos that had a historical range across Java, Sumatra, Southeast Asia, all the way through China into India.
Sumatran tigers are a tiger subspecies found in the Indonesian island of Sumatra. They are the smallest of all tigers, being comparable in size to a l. The African Bush Elephant is the largest and heaviest of the land mammals in the world. They are scattered roughly throughout the savanna region of th.
The Hooded Seal, also called the Bladder-nosed Seal, is a species of large silver-grey seal found in the cold waters of the Atlantic. The seal gets it. Reproduction in whole or in part without permission is prohibited.
Mammal Classification At present, there are more than 5, extant species belonging to the Mammalian class. Following is the general classification of the animals: Family Ornithorhynchidae the duck-billed platypus Family Tachyglossidae the spiny anteaters.
Evolution of Mammals Amniotes were the first vertebrates to be fully terrestrial. Anatomy of Mammals Skeletal System Majority of these animals, including giraffes, bats, whales and humans, have 7 cervical vertebrae neck bones. Respiratory system Animals belonging to this class respire by inhaling oxygen and exhaling carbon dioxide.
Nervous system The brains of the placental mammals possess a corpus callosum, which is absent in monotremes and marsupials. Skin The Integumentary system or skin is comprised of three layers: Circulatory System These animals have four-chambered hearts with separate right and left ventricles. Digestive System Carnivorous Mammals have a simple digestive system because the lipids, proteins and minerals found in their diet are very easy to digest.
The main components of the digestive system include: Oral cavity Pharynx Esophagus Stomach Small intestine Large intestine It has an alimentary canal and different accessory glands that help in digestion by secreting digestive fluids. Excretory System The excretory system in most of the species consists of two kidneys, a renal artery, nephrons and glomeruli. Characteristics of Mammals Modern Mammals have many characteristic features that help to distinguish them from other amniotes and also the amphibians: The presence of body hair is a unique characteristic of Mammals.
Animals from no other class have true hair, while all mammals are at least partially covered with hair at some stage in their lives. For example, dolphins and whales are born with mustaches that fall out as they become adults. The females of all Mammal species have mammary glands that produce milk for nursing the offspring.
Mammary glands are actually modified sweat glands, which are also a distinctive feature present in these animals. This is the typical tooth replacement pattern seen in Mammalians. In this pattern, tooth replacement occurs only once in the entire lifetime of the animals. The juveniles have smaller and weaker teeth compared to the adult teeth. It is called the deciduous teeth which fall out to be replaced by the stronger and larger permanent teeth as the animals grow up. This refers to the ability to control one's own body temperature and keep it relatively constant regardless of the outside temperature.
All mammals are Endothermic or warm-blooded. Their hearts are distinctively separated into four chambers, 2 atria and 2 ventricles. This feature helps to distinguish them from reptiles and amphibians. However, birds also have four-chambered hearts. The presence of the neocortex region in the brain is a unique feature of these animals. The following anatomical traits are observed for this purpose: A single bone in the lower jaw: Another characteristic feature seen in the Mammalian species is the single bone in their lower jaw which is directly attached to the skull.
This bone is known as the dentary as it holds the lower jaw teeth. Three bones in middle ear: The unique arrangement of three middle ear bones, known as the incus anvil , malleus hammer and stapes stirrup is another important characteristic. In Mammals, the dentary lower jaw bone holding the teeth and the squamosal a small skull bone converge to form the jaw joint. Included in the carbohydrate group are sugars, starches, cellulose, and various other substances.
Glucose is a simple sugar produced in cells by the breakdown of more complex carbohydrates, including starch, cellulose, and such complex sugars as sucrose cane or beet sugar and fructose fruit sugar. In cellular respiration, an organism oxidizes glucose i. ATP, critical to metabolism the breakdown of nutrients to provide energy or form new material , is the compound used by cells to carry out most of their ordinary functions. Among those functions are the production of new cell parts and chemicals, the movement of compounds through cells and the body as a whole, and growth.
In cellular respiration, six molecules of glucose C 6 H 12 O 6 react with six molecules of oxygen O 2 to form six molecules of carbon dioxide CO 2 , six molecules of water H 2 O , and 36 molecules of ATP. This can be represented by the following chemical equation:. The process is much more complicated than this equation makes it appear: All animals have some mechanism for removing oxygen from the air and transmitting it into the bloodstream, and this same mechanism typically is used to expel carbon dioxide from the bloodstream into the surrounding environment.
Types of animal respiration, in order of complexity, include direct diffusion, diffusion into blood, tracheal respiration, respiration with gills, and finally, respiration through lungs. Microbes, fungi, and plants all obtain the oxygen they use for cellular respiration directly from the environment, meaning that there are no intermediate organs or bodily chemicals, such as lungs or blood.
More complex organisms, such as sponges, jellyfish, and terrestrial land flatworms, all of which have blood, also breathe through direct diffusion. The latter term describes an exchange of oxygen and carbon dioxide directly between an organism, or its bloodstream, and the surrounding environment. Once oxygen is in the blood, it moves throughout the body to different tissues and cells.
Among the organisms that rely on diffusion into blood are annelids, a group that includes earthworms, various marine worms, and leeches. In tracheal respiration air moves through openings in the body surface called spiracles.
It then passes into special breathing tubes called tracheae that extend into the body. The tracheae divide into many small branches that are in contact with muscles and organs. In small insects, air simply moves into the tracheae, while in large insects, body movements assist tracheal air movement.
Insects and terrestrial arthropods land-based organisms with external skeletons use this method of respiration. Much more complicated than tracheae, gills are specialized tissues with many infoldings.
Each gill is covered by a thin layer of cells and filled with blood capillaries. These capillaries take up oxygen dissolved in water and expel carbon dioxide dissolved in blood. Fish and other aquatic animals use gills, as did the early ancestors of humans and other higher animals. A remnant of this chapter from humans' evolutionary history can be seen in the way that an embryo breathes in its mother's womb, not by drawing in oxygen through its lungs but through gill-like mechanisms that disappear as the embryo develops.
Lungs are composed of many small chambers or air sacs surrounded by blood capillaries. Thus, they work with the circulatory system, which transports oxygen from inhaled air to all tissues of the body and also transports carbon dioxide from body cells to the lungs to be exhaled. After air enters the lungs, oxygen moves into the bloodstream through the walls of these capillaries.
It then passes from the lung capillaries to the different muscles and organs of the body. Although they are common to amphibians, reptiles, birds, and mammals, lungs differ enormously throughout the animal kingdom. Frogs, for instance, have balloon-like lungs that do not have a very large surface area. By contrast, if the entire surface of an adult male human's lungs were spread flat, it would cover about sq. The reason is that humans have about million gas-filled alveoli, tiny protrusions inside the lungs that greatly expand the surface area for gas exchange.
Birds have specialized lungs that use a mechanism called crosscurrent exchange, which allows air to flow in one direction only, making for more efficient oxygen exchange. They have some eight thin-walled air sacs attached to their lungs, and when they inhale, air passes through a tube called the bronchus and enters posterior air sacs—that is, sacs located toward the rear.
At the same time, air in the lungs moves forward to anterior air sacs, or ones located near the bird's front. When the bird exhales, air from the rear air sacs moves to the outside environment, while air from the front moves into the lungs.
This efficient system moves air forward through the lungs when the bird inhales and exhales and makes it possible for birds to fly at high altitudes, where the air has a low oxygen content. Humans and other mammals have lungs in which air moves in and out through the same pathway. This is true even of dolphins and whales, though they differ from humans in that they do not take in nutrition through the same opening.
In fact, terrestrial mammals, such as the human, horse, or dog, are some of the only creatures that possess two large respiratory openings: Activity that involves oxygen is called aerobic; hence the term aerobic exercise, which refers to running, calisthenics, biking, or any other form of activity that increases the heart rate and breathing.
Activity that does not involve oxygen intake is called anaerobic. Weightlifting, for instance, will increase the heart rate and rate of breathing if it is done intensely, but that is not its purpose and it does not depend on the intake and outflow of breath. For that reason, it is called an anaerobic exercise—though, obviously, a person has to keep breathing while doing it. In fact, a person cannot consciously stop breathing for a prolonged period, and for this reason, people cannot kill themselves simply by holding their breath.
A buildup of carbon dioxide and hydrogen ions electrically charged atoms in the bloodstream stimulates the breathing centers to become active, no matter what we try to do. On the other hand, if a person were underwater, the lungs would draw in water instead of air, and though water contains air, the drowning person would suffocate.
Some creatures, however, do not need to breathe air but instead survive by anaerobic respiration. This is true primarily of some forms of bacteria, and indeed scientists believe that the first organisms to appear on Earth's surface were anaerobic.
Those organisms arose when Earth's atmosphere contained very little oxygen, and as the composition of the atmosphere began to incorporate more oxygen over the course of many millions of years, new organisms evolved that were adapted to that condition.
The essay on paleontology discusses Earth's early history, including the existence of anaerobic life before the formation of oxygen in the atmosphere. The appearance of oxygen is a result of plant life, which produces it as a byproduct of the conversion of carbon dioxide that takes place in photosynthesis. Plants, therefore, are technically anaerobic life-forms, though that term usually refers to types of bacteria that neither inhale nor exhale oxygen. Anaerobic bacteria still exist on Earth and serve humans in many ways.
Some play a part in the production of foods, as in the process of fermentation. Other anaerobic bacteria have a role in the treatment of sewage. Living in an environment that would kill most creatures—and not just because of the lack of oxygen—they consume waste materials, breaking them down chemically into simpler compounds.
Even in creatures, such as humans, that depend on aerobic respiration, anaerobic respiration can take place. Most cells are able to switch from aerobic to anaerobic respiration when necessary, but they generally are not able to continue producing energy by this process for very long. For example, a person who exercises vigorously may be burning up glucose faster than oxygen is being pumped to the cells, meaning that cellular respiration cannot take place quickly enough to supply all the energy the body needs.
One advantage of anaerobic respiration is that it can take place very quickly and in short bursts, as opposed to aerobic respiration, which is designed for slower and steadier use of muscles. The disadvantage is that anaerobic respiration produces lactic acid, which, when it builds up in muscles that are overworked, causes soreness and may even lead to cramps. Eventually, the buildup of lactic acid is carried away in the bloodstream, and the lactic acid is converted to carbon dioxide and water vapor, both of which are exhaled.
But if lactic acid levels in the bloodstream rise faster than the body can neutralize them, a state known as lactic acidosis may ensue. Lactic acidosis rarely happens in healthy people and, more often than not, is a result of the body's inability to obtain sufficient oxygen, as occurs in heart attacks or carbon monoxide or cyanide poisoning or in the context of diseases such as diabetes.
They are superficially similar to lizards but, along with mammals and birds, reptiles are amniotes and do not require water bodies in which to breed. With their complex reproductive needs and permeable skins, amphibians are often ecological indicators ; in recent decades there has been a dramatic decline in amphibian populations for many species around the globe.
The earliest amphibians evolved in the Devonian period from sarcopterygian fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land. They diversified and became dominant during the Carboniferous and Permian periods, but were later displaced by reptiles and other vertebrates.
Over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass Lissamphibia. The three modern orders of amphibians are Anura the frogs and toads , Urodela the salamanders , and Apoda the caecilians. The smallest amphibian and vertebrate in the world is a frog from New Guinea Paedophryne amauensis with a length of just 7. The largest living amphibian is the 1.
The study of amphibians is called batrachology , while the study of both reptiles and amphibians is called herpetology. The term was initially used as a general adjective for animals that could live on land or in water, including seals and otters.
Amphibia in its widest sense sensu lato was divided into three subclasses , two of which are extinct: The actual number of species in each group depends on the taxonomic classification followed. The two most common systems are the classification adopted by the website AmphibiaWeb, University of California, Berkeley and the classification by herpetologist Darrel Frost and the American Museum of Natural History , available as the online reference database "Amphibian Species of the World".
With the phylogenetic classification, the taxon Labyrinthodontia has been discarded as it is a polyparaphyletic group without unique defining features apart from shared primitive characteristics. Classification varies according to the preferred phylogeny of the author and whether they use a stem-based or a node-based classification. Traditionally, amphibians as a class are defined as all tetrapods with a larval stage, while the group that includes the common ancestors of all living amphibians frogs, salamanders and caecilians and all their descendants is called Lissamphibia.
The phylogeny of Paleozoic amphibians is uncertain, and Lissamphibia may possibly fall within extinct groups, like the Temnospondyli traditionally placed in the subclass Labyrinthodontia or the Lepospondyli, and in some analyses even in the amniotes.
This means that advocates of phylogenetic nomenclature have removed a large number of basal Devonian and Carboniferous amphibian-type tetrapod groups that were formerly placed in Amphibia in Linnaean taxonomy , and included them elsewhere under cladistic taxonomy.
All modern amphibians are included in the subclass Lissamphibia, which is usually considered a clade , a group of species that have evolved from a common ancestor. The three modern orders are Anura the frogs and toads , Caudata or Urodela, the salamanders , and Gymnophiona or Apoda, the caecilians.
It is anatomically very similar to modern frogs. Authorities disagree as to whether Salientia is a superorder that includes the order Anura, or whether Anura is a sub-order of the order Salientia. The Lissamphibia are traditionally divided into three orders , but an extinct salamander-like family, the Albanerpetontidae , is now considered part of Lissamphibia alongside the superorder Salientia. Furthermore, Salientia includes all three recent orders plus the Triassic proto-frog, Triadobatrachus.
The first major groups of amphibians developed in the Devonian period, around million years ago, from lobe-finned fish which were similar to the modern coelacanth and lungfish. Some fish had developed primitive lungs to help them breathe air when the stagnant pools of the Devonian swamps were low in oxygen.
They could also use their strong fins to hoist themselves out of the water and onto dry land if circumstances so required.
Eventually, their bony fins would evolve into limbs and they would become the ancestors to all tetrapods , including modern amphibians, reptiles, birds, and mammals. Despite being able to crawl on land, many of these prehistoric tetrapodomorph fish still spent most of their time in the water.
They had started to develop lungs, but still breathed predominantly with gills. Many examples of species showing transitional features have been discovered. Ichthyostega was one of the first primitive amphibians, with nostrils and more efficient lungs.
It had four sturdy limbs, a neck, a tail with fins and a skull very similar to that of the lobe-finned fish, Eusthenopteron. Their lungs improved and their skeletons became heavier and stronger, better able to support the weight of their bodies on land. They developed "hands" and "feet" with five or more digits;  the skin became more capable of retaining body fluids and resisting desiccation.
At the end of the Devonian period million years ago , the seas, rivers and lakes were teeming with life while the land was the realm of early plants and devoid of vertebrates,  though some, such as Ichthyostega , may have sometimes hauled themselves out of the water.
It is thought they may have propelled themselves with their forelimbs, dragging their hindquarters in a similar manner to that used by the elephant seal. Extensive swamps developed with mosses , ferns , horsetails and calamites. Air-breathing arthropods evolved and invaded the land where they provided food for the carnivorous amphibians that began to adapt to the terrestrial environment.
There were no other tetrapods on the land and the amphibians were at the top of the food chain, occupying the ecological position currently held by the crocodile. Though equipped with limbs and the ability to breathe air, most still had a long tapering body and strong tail. They still needed to return to water to lay their shell-less eggs, and even most modern amphibians have a fully aquatic larval stage with gills like their fish ancestors. It was the development of the amniotic egg, which prevents the developing embryo from drying out, that enabled the reptiles to reproduce on land and which led to their dominance in the period that followed.
After the Carboniferous rainforest collapse amphibian dominance gave way to reptiles,  and amphibians were further devastated by the Permian—Triassic extinction event. According to the fossil record, Lissamphibia , which includes all modern amphibians and is the only surviving lineage, may have branched off from the extinct groups Temnospondyli and Lepospondyli at some period between the Late Carboniferous and the Early Triassic.
The origins and evolutionary relationships between the three main groups of amphibians is a matter of debate. A molecular phylogeny, based on rDNA analysis, suggests that salamanders and caecilians are more closely related to each other than they are to frogs.
It also appears that the divergence of the three groups took place in the Paleozoic or early Mesozoic around million years ago , before the breakup of the supercontinent Pangaea and soon after their divergence from the lobe-finned fish.
The briefness of this period, and the swiftness with which radiation took place, would help account for the relative scarcity of primitive amphibian fossils. As they evolved from lunged fish, amphibians had to make certain adaptations for living on land, including the need to develop new means of locomotion. In the water, the sideways thrusts of their tails had propelled them forward, but on land, quite different mechanisms were required.
Their vertebral columns, limbs, limb girdles and musculature needed to be strong enough to raise them off the ground for locomotion and feeding. Terrestrial adults discarded their lateral line systems and adapted their sensory systems to receive stimuli via the medium of the air.
They needed to develop new methods to regulate their body heat to cope with fluctuations in ambient temperature. They developed behaviours suitable for reproduction in a terrestrial environment. Their skins were exposed to harmful ultraviolet rays that had previously been absorbed by the water. The skin changed to become more protective and prevent excessive water loss.
The superclass Tetrapoda is divided into four classes of vertebrate animals with four limbs. The smallest amphibian and vertebrate in the world is a microhylid frog from New Guinea Paedophryne amauensis first discovered in It has an average length of 7. Amphibians are ectothermic cold-blooded vertebrates that do not maintain their body temperature through internal physiological processes.
Their metabolic rate is low and as a result, their food and energy requirements are limited. In the adult state, they have tear ducts and movable eyelids, and most species have ears that can detect airborne or ground vibrations. They have muscular tongues, which in many species can be protruded. Modern amphibians have fully ossified vertebrae with articular processes.
Their ribs are usually short and may be fused to the vertebrae. Their skulls are mostly broad and short, and are often incompletely ossified. Their skin contains little keratin and lacks scales, apart from a few fish-like scales in certain caecilians.
The skin contains many mucous glands and in some species, poison glands a type of granular gland. The hearts of amphibians have three chambers, two atria and one ventricle. They have a urinary bladder and nitrogenous waste products are excreted primarily as urea. Most amphibians lay their eggs in water and have aquatic larvae that undergo metamorphosis to become terrestrial adults. Amphibians breathe by means of a pump action in which air is first drawn into the buccopharyngeal region through the nostrils.
These are then closed and the air is forced into the lungs by contraction of the throat. The order Anura from the Ancient Greek a n - meaning "without" and oura meaning "tail" comprises the frogs and toads.
They usually have long hind limbs that fold underneath them, shorter forelimbs, webbed toes with no claws, no tails, large eyes and glandular moist skin.
The difference is not a formal one taxonomically and there are numerous exceptions to this rule. Members of the family Bufonidae are known as the "true toads". They are found worldwide except for polar areas. Anura is divided into three suborders that are broadly accepted by the scientific community, but the relationships between some families remain unclear.
Future molecular studies should provide further insights into their evolutionary relationships. These are Ascaphidae , Bombinatoridae , Discoglossidae and Leiopelmatidae which have few derived features and are probably paraphyletic with regard to other frog lineages.
These have certain characteristics that are intermediate between the two other suborders. Ninety-six percent of the over 5, extant species of frog are neobatrachians. The order Caudata from the Latin cauda meaning "tail" consists of the salamanders—elongated, low-slung animals that mostly resemble lizards in form. This is a symplesiomorphic trait and they are no more closely related to lizards than they are to mammals. They range in size from the Chinese giant salamander Andrias davidianus , which has been reported to grow to a length of 1.
The family Plethodontidae is also found in Central America and South America north of the Amazon basin ;  South America was apparently invaded from Central America by about the start of the Miocene , 23 million years ago. They may be terrestrial or aquatic and many spend part of the year in each habitat. When on land, they mostly spend the day hidden under stones or logs or in dense vegetation, emerging in the evening and night to forage for worms, insects and other invertebrates.
The suborder Cryptobranchoidea contains the primitive salamanders. A number of fossil cryptobranchids have been found, but there are only three living species, the Chinese giant salamander Andrias davidianus , the Japanese giant salamander Andrias japonicus and the hellbender Cryptobranchus alleganiensis from North America.
These large amphibians retain several larval characteristics in their adult state; gills slits are present and the eyes are unlidded. A unique feature is their ability to feed by suction, depressing either the left side of their lower jaw or the right. As well as breathing with lungs, they respire through the many folds in their thin skin, which has capillaries close to the surface.
The suborder Salamandroidea contains the advanced salamanders. They differ from the cryptobranchids by having fused prearticular bones in the lower jaw, and by using internal fertilisation. In salamandrids, the male deposits a bundle of sperm, the spermatophore , and the female picks it up and inserts it into her cloaca where the sperm is stored until the eggs are laid.
The family Salamandridae includes the true salamanders and the name " newt " is given to members of its subfamily Pleurodelinae. The third suborder, Sirenoidea , contains the four species of sirens, which are in a single family, Sirenidae. Members of this order are eel -like aquatic salamanders with much reduced forelimbs and no hind limbs. Some of their features are primitive while others are derived. Despite this, the eggs are laid singly, a behaviour not conducive for external fertilisation.
The order Gymnophiona from the Greek gymnos meaning "naked" and ophis meaning "serpent" or Apoda from the Latin an- meaning "without" and the Greek poda meaning "legs" comprises the caecilians. These are long, cylindrical, limbless animals with a snake- or worm-like form.
The adults vary in length from 8 to 75 centimetres 3 to 30 inches with the exception of Thomson's caecilian Caecilia thompsoni , which can reach centimetres 4. A caecilian's skin has a large number of transverse folds and in some species contains tiny embedded dermal scales.
It has rudimentary eyes covered in skin, which are probably limited to discerning differences in light intensity. It also has a pair of short tentacles near the eye that can be extended and which have tactile and olfactory functions.
Most caecilians live underground in burrows in damp soil, in rotten wood and under plant debris, but some are aquatic. Others brood their eggs and the larvae undergo metamorphosis before the eggs hatch. A few species give birth to live young, nourishing them with glandular secretions while they are in the oviduct. The integumentary structure contains some typical characteristics common to terrestrial vertebrates, such as the presence of highly cornified outer layers, renewed periodically through a moulting process controlled by the pituitary and thyroid glands.
Local thickenings often called warts are common, such as those found on toads. The outside of the skin is shed periodically mostly in one piece, in contrast to mammals and birds where it is shed in flakes. Amphibians often eat the sloughed skin. The similarity of these to the scales of bony fish is largely superficial.
Lizards and some frogs have somewhat similar osteoderms forming bony deposits in the dermis, but this is an example of convergent evolution with similar structures having arisen independently in diverse vertebrate lineages. Amphibian skin is permeable to water.
Gas exchange can take place through the skin cutaneous respiration and this allows adult amphibians to respire without rising to the surface of water and to hibernate at the bottom of ponds.
The secretions produced by these help keep the skin moist. In addition, most species of amphibian have granular glands that secrete distasteful or poisonous substances. Some amphibian toxins can be lethal to humans while others have little effect. The skin colour of amphibians is produced by three layers of pigment cells called chromatophores.
These three cell layers consist of the melanophores occupying the deepest layer , the guanophores forming an intermediate layer and containing many granules, producing a blue-green colour and the lipophores yellow, the most superficial layer.
The colour change displayed by many species is initiated by hormones secreted by the pituitary gland. Unlike bony fish, there is no direct control of the pigment cells by the nervous system, and this results in the colour change taking place more slowly than happens in fish.
A vividly coloured skin usually indicates that the species is toxic and is a warning sign to predators. Amphibians have a skeletal system that is structurally homologous to other tetrapods, though with a number of variations. They all have four limbs except for the legless caecilians and a few species of salamander with reduced or no limbs.
The bones are hollow and lightweight. The musculoskeletal system is strong to enable it to support the head and body. The bones are fully ossified and the vertebrae interlock with each other by means of overlapping processes. The pectoral girdle is supported by muscle, and the well-developed pelvic girdle is attached to the backbone by a pair of sacral ribs. The ilium slopes forward and the body is held closer to the ground than is the case in mammals. In most amphibians, there are four digits on the fore foot and five on the hind foot, but no claws on either.
Some salamanders have fewer digits and the amphiumas are eel-like in appearance with tiny, stubby legs. The sirens are aquatic salamanders with stumpy forelimbs and no hind limbs. The caecilians are limbless. They burrow in the manner of earthworms with zones of muscle contractions moving along the body.
On the surface of the ground or in water they move by undulating their body from side to side. In frogs, the hind legs are larger than the fore legs, especially so in those species that principally move by jumping or swimming. In the walkers and runners the hind limbs are not so large, and the burrowers mostly have short limbs and broad bodies.
The feet have adaptations for the way of life, with webbing between the toes for swimming, broad adhesive toe pads for climbing, and keratinised tubercles on the hind feet for digging frogs usually dig backwards into the soil. In most salamanders, the limbs are short and more or less the same length and project at right angles from the body. Locomotion on land is by walking and the tail often swings from side to side or is used as a prop, particularly when climbing.
In their normal gait, only one leg is advanced at a time in the manner adopted by their ancestors, the lobe-finned fish. Adult frogs do not have tails and caecilians have only very short ones. Salamanders use their tails in defence and some are prepared to jettison them to save their lives in a process known as autotomy. Certain species in the Plethodontidae have a weak zone at the base of the tail and use this strategy readily.
The tail often continues to twitch after separation which may distract the attacker and allow the salamander to escape. Both tails and limbs can be regenerated.
Amphibians have a juvenile stage and an adult stage, and the circulatory systems of the two are distinct. In the juvenile or tadpole stage, the circulation is similar to that of a fish; the two-chambered heart pumps the blood through the gills where it is oxygenated, and is spread around the body and back to the heart in a single loop.
In the adult stage, amphibians especially frogs lose their gills and develop lungs. They have a heart that consists of a single ventricle and two atria.
When the ventricle starts contracting, deoxygenated blood is pumped through the pulmonary artery to the lungs. Continued contraction then pumps oxygenated blood around the rest of the body. Mixing of the two bloodstreams is minimized by the anatomy of the chambers. The nervous system is basically the same as in other vertebrates, with a central brain, a spinal cord, and nerves throughout the body. The amphibian brain is less well developed than that of reptiles, birds and mammals but is similar in morphology and function to that of a fish.
It is believed amphibians are capable of perceiving pain. The brain consists of equal parts, cerebrum , midbrain and cerebellum. Various parts of the cerebrum process sensory input, such as smell in the olfactory lobe and sight in the optic lobe, and it is additionally the centre of behaviour and learning. The cerebellum is the center of muscular coordination and the medulla oblongata controls some organ functions including heartbeat and respiration.
The brain sends signals through the spinal cord and nerves to regulate activity in the rest of the body. The pineal body , known to regulate sleep patterns in humans, is thought to produce the hormones involved in hibernation and aestivation in amphibians.
Tadpoles retain the lateral line system of their ancestral fishes, but this is lost in terrestrial adult amphibians. Some caecilians possess electroreceptors that allow them to locate objects around them when submerged in water. The ears are well developed in frogs.
There is no external ear, but the large circular eardrum lies on the surface of the head just behind the eye. This vibrates and sound is transmitted through a single bone, the stapes , to the inner ear. Only high-frequency sounds like mating calls are heard in this way, but low-frequency noises can be detected through another mechanism.
Another feature, unique to frogs and salamanders, is the columella-operculum complex adjoining the auditory capsule which is involved in the transmission of both airborne and seismic signals. The eyes of tadpoles lack lids, but at metamorphosis, the cornea becomes more dome-shaped, the lens becomes flatter, and eyelids and associated glands and ducts develop. They allow colour vision and depth of focus. In the retinas are green rods, which are receptive to a wide range of wavelengths.
Many amphibians catch their prey by flicking out an elongated tongue with a sticky tip and drawing it back into the mouth before seizing the item with their jaws.
Some use inertial feeding to help them swallow the prey, repeatedly thrusting their head forward sharply causing the food to move backwards in their mouth by inertia. Most amphibians swallow their prey whole without much chewing so they possess voluminous stomachs. The short oesophagus is lined with cilia that help to move the food to the stomach and mucus produced by glands in the mouth and pharynx eases its passage.
The enzyme chitinase produced in the stomach helps digest the chitinous cuticle of arthropod prey. Amphibians possess a pancreas , liver and gall bladder.
The liver is usually large with two lobes. Its size is determined by its function as a glycogen and fat storage unit, and may change with the seasons as these reserves are built or used up.