Tetrapods are four-limbed animals constituting the superclass Tetrapoda. It includes existing and extinct amphibians and mammals. Tetrapods evolved from a group of animals known as the Tetrapodomorpha which, in turn, evolved from ancient Sarcopterygii around 390 million years ago in the middle Devonian period; the first tetrapods appeared by the late Devonian, 367.5 million years ago. The change from a body plan for breathing and navigating in water to a body plan enabling the animal to move on land is one of the most profound evolutionary changes known; the first tetrapods were aquatic. Modern amphibians, which evolved from earlier groups, are semiaquatic. However, most tetrapod species today are amniotes, most of those are terrestrial tetrapods whose branch evolved from earlier tetrapods about 340 million years ago; the key innovation in amniotes over amphibians is laying of eggs on land or having further evolved to retain the fertilized egg within the mother. Amniote tetrapods drove most amphibian tetrapods to extinction.
One group of amniotes diverged into the reptiles, which includes lepidosaurs, crocodilians and extinct relatives. Amniotes include the tetrapods that further evolved for flight—such as birds from among the dinosaurs, bats from among the mammals; some tetrapods, such as the snakes, have lost some or all of their limbs through further speciation and evolution. Others, such as amphibians, returned to or aquatic lives, the first during the Carboniferous period. Tetrapods have numerous anatomical and physiological features that are distinct from their aquatic ancestors; these include the structure of the jaw and teeth for feeding on land, limb girdles and extremities for land locomotion, lungs for respiration in air, a heart for circulation, eyes and ears for seeing and hearing in air. Tetrapods can be defined in cladistics as the nearest common ancestor of all living amphibians and all living amniotes, along with all of the descendants of that ancestor; this is a node-based definition. The group so defined is crown tetrapods.
The term tetrapodomorph is used for the stem-based definition: any animal, more related to living amphibians, reptiles and mammals than to living dipnoi. The group so defined is known as the tetrapod total group. Stegocephalia is a larger group equivalent to some broader uses of the word tetrapod, used by scientists who prefer to reserve tetrapod for the crown group; such scientists use the term "stem-tetrapod" to refer to those tetrapod-like vertebrates that are not members of the crown group, including the tetrapodomorph fishes. The two subclades of crown tetrapods are Reptiliomorpha. Batrachomorphs are all animals sharing a more recent common ancestry with living amphibians than with living amniotes. Reptiliomorphs are all animals sharing a more recent common ancestry with living amniotes than with living amphibians. Tetrapoda includes four living classes: amphibians, reptiles and birds. Overall, the biodiversity of lissamphibians, as well as of tetrapods has grown exponentially over time. However, that diversification process was interrupted at least a few times by major biological crises, such as the Permian–Triassic extinction event, which at least affected amniotes.
The overall composition of biodiversity was driven by amphibians in the Palaeozoic, dominated by reptiles in the Mesozoic and expanded by the explosive growth of birds and mammals in the Cenozoic. As biodiversity has grown, so has the number of niches that tetrapods have occupied; the first tetrapods were aquatic and fed on fish. Today, the Earth supports a great diversity of tetrapods that live in many habitats and subsist on a variety of diets; the following table shows summary estimates for each tetrapod class from the IUCN Red List of Threatened Species, 2014.3, for the number of extant species that have been described in the literature, as well as the number of threatened species. The classification of tetrapods has a long history. Traditionally, tetrapods are divided into four classes based on gross anatomical and physiological traits. Snakes and other legless reptiles are considered tetrapods because they are sufficiently like other reptiles that have a full complement of limbs. Similar considerations apply to aquatic mammals.
Newer taxonomy is based on cladistics instead, giving a variable number of major "branches" of the tetrapod family tree. As is the case throughout evolutionary biology today, there is debate over how to properly classify the groups within Tetrapoda. Traditional biological classification sometimes fa
Harvard University is a private Ivy League research university in Cambridge, with about 6,700 undergraduate students and about 15,250 postgraduate students. Established in 1636 and named for its first benefactor, clergyman John Harvard, Harvard is the United States' oldest institution of higher learning, its history and wealth have made it one of the world's most prestigious universities; the Harvard Corporation is its first chartered corporation. Although never formally affiliated with any denomination, the early College trained Congregational and Unitarian clergy, its curriculum and student body were secularized during the 18th century, by the 19th century, Harvard had emerged as the central cultural establishment among Boston elites. Following the American Civil War, President Charles W. Eliot's long tenure transformed the college and affiliated professional schools into a modern research university. A. Lawrence Lowell, who followed Eliot, further reformed the undergraduate curriculum and undertook aggressive expansion of Harvard's land holdings and physical plant.
James Bryant Conant led the university through the Great Depression and World War II and began to reform the curriculum and liberalize admissions after the war. The undergraduate college became coeducational after its 1977 merger with Radcliffe College; the university is organized into eleven separate academic units—ten faculties and the Radcliffe Institute for Advanced Study—with campuses throughout the Boston metropolitan area: its 209-acre main campus is centered on Harvard Yard in Cambridge 3 miles northwest of Boston. Harvard's endowment is worth $39.2 billion, making it the largest of any academic institution. Harvard is a large residential research university; the nominal cost of attendance is high, but the university's large endowment allows it to offer generous financial aid packages. The Harvard Library is the world's largest academic and private library system, comprising 79 individual libraries holding over 18 million items; the University is cited as one of the world's top tertiary institutions by various organizations.
Harvard's alumni include eight U. S. presidents, more than thirty foreign heads of state, 62 living billionaires, 359 Rhodes Scholars, 242 Marshall Scholars. As of October 2018, 158 Nobel laureates, 18 Fields Medalists, 14 Turing Award winners have been affiliated as students, faculty, or researchers. In addition, Harvard students and alumni have won 10 Academy Awards, 48 Pulitzer Prizes and 108 Olympic medals, have founded a large number of companies worldwide. Harvard was established in 1636 by vote of the Great and General Court of the Massachusetts Bay Colony. In 1638, it acquired British North America's first known printing press. In 1639, it was named Harvard College after deceased clergyman John Harvard, an alumnus of the University of Cambridge, who had left the school £779 and his scholar's library of some 400 volumes; the charter creating the Harvard Corporation was granted in 1650. A 1643 publication gave the school's purpose as "to advance learning and perpetuate it to posterity, dreading to leave an illiterate ministry to the churches when our present ministers shall lie in the dust".
It offered a classic curriculum on the English university model—many leaders in the colony had attended the University of Cambridge—but conformed to the tenets of Puritanism. It was never affiliated with any particular denomination, but many of its earliest graduates went on to become clergymen in Congregational and Unitarian churches; the leading Boston divine Increase Mather served as president from 1685 to 1701. In 1708, John Leverett became the first president, not a clergyman, marking a turning of the college from Puritanism and toward intellectual independence. Throughout the 18th century, Enlightenment ideas of the power of reason and free will became widespread among Congregational ministers, putting those ministers and their congregations in tension with more traditionalist, Calvinist parties; when the Hollis Professor of Divinity David Tappan died in 1803 and the president of Harvard Joseph Willard died a year in 1804, a struggle broke out over their replacements. Henry Ware was elected to the chair in 1805, the liberal Samuel Webber was appointed to the presidency of Harvard two years which signaled the changing of the tide from the dominance of traditional ideas at Harvard to the dominance of liberal, Arminian ideas.
In 1846, the natural history lectures of Louis Agassiz were acclaimed both in New York and on the campus at Harvard College. Agassiz's approach was distinctly idealist and posited Americans' "participation in the Divine Nature" and the possibility of understanding "intellectual existences". Agassiz's perspective on science combined observation with intuition and the assumption that a person can grasp the "divine plan" in all phenomena; when it came to explaining life-forms, Agassiz resorted to matters of shape based on a presumed archetype for his evidence. This dual view of knowledge was in concert with the teachings of Common Sense Realism derived from Scottish philosophers Thomas Reid and Dugald Stewart, whose works were part of the Harvard curriculum at the time; the popularity of Agassiz's efforts to "soar with Plato" also derived from other writings to which Harvard students
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
Undulatory locomotion is the type of motion characterized by wave-like movement patterns that act to propel an animal forward. Examples of this type of gait include swimming in the lamprey. Although this is the type of gait utilized by limbless animals, some creatures with limbs, such as the salamander, choose to forgo use of their legs in certain environments and exhibit undulatory locomotion; this movement strategy is important to study in order to create novel robotic devices capable of traversing a variety of environments. In limbless locomotion, forward locomotion is generated by propagating flexural waves along the length of the animal's body. Forces generated between the animal and surrounding environment lead to a generation of alternating sideways forces that act to move the animal forward; these forces generate drag. Simulation predicts that thrust and drag are dominated by viscous forces at low Reynolds numbers and inertial forces at higher Reynolds numbers; when the animal swims in a fluid, two main forces are thought to play a role: Skin Friction: Generated due to the resistance of a fluid to shearing and is proportional to speed of the flow.
This dominates undulatory swimming in spermatozoa and the nematode Form Force: Generated by the differences in pressure on the surface of the body and it varies with the square of flow speed. At low Reynolds number, skin friction accounts for nearly all of the drag. For those animals which undulate at intermediate Reynolds number, such as the Ascidian larvae, both skin friction and form force account for the production of drag and thrust. At high Reynolds number, both skin friction and form force act to generate drag, but only form force produces thrust. In animals that move without use of limbs, the most common feature of the locomotion is a rostral to caudal wave that travels down their body. However, this pattern can change based on the particular undulating animal, the environment, the metric in which the animal is optimizing; the most common mode of motion is simple undulations in which lateral bending is propagated from head to tail. Snakes can exhibit 5 different modes of terrestrial locomotion: lateral undulation, concertina and slide-pushing.
Lateral undulation resembles the simple undulatory motion observed in many other animals such as in lizards and fish, in which waves of lateral bending propagate down the snake's body. The American eel moves in an aquatic environment, though it can move on land for short periods of time, it is able to move about in both environments by producing traveling waves of lateral undulations. However, differences between terrestrial and aquatic locomotor strategy suggest that the axial musculature is being activated differently. In terrestrial locomotion, all points along the body move on the same path and, the lateral displacements along the length of the eel's body is the same. However, in aquatic locomotion, different points along the body follow different paths with increasing lateral amplitude more posteriorly. In general, the amplitude of the lateral undulation and angle of intervertebral flexion is much greater during terrestrial locomotion than that of aquatic. A typical characteristic of many animals that utilize undulatory locomotion is that they have segmented muscles, or blocks of myomeres, running from their head to tails which are separated by connective tissue called myosepta.
In addition, some segmented muscle groups, such as the lateral hypaxial musculature in the salamander are oriented at an angle to the longitudinal direction. For these obliquely oriented fibers the strain in the longitudinal direction is greater than the strain in the muscle fiber direction leading to an architectural gear ratio greater than 1. A higher initial angle of orientation and more dorsoventral bulging produces a faster muscle contraction but results in a lower amount of force production, it is hypothesized that animals employ a variable gearing mechanism that allows self-regulation of force and velocity to meet the mechanical demands of the contraction. When a pennate muscle is subjected to a low force, resistance to width changes in the muscle cause it to rotate which produces a higher architectural gear ratio. However, when subject to a high force, the perpendicular fiber force component overcomes the resistance to width changes and the muscle compresses producing a lower AGR. Most fishes bend as a simple, homogenous beam during swimming via contractions of longitudinal red muscle fibers and obliquely oriented white muscle fibers within the segmented axial musculature.
The fiber strain experienced by the longitudinal red muscle fibers is equivalent to the longitudinal strain. The deeper white muscle fibers fishes show diversity in arrangement; these fibers are organized into cone-shaped structures and attach to connective tissue sheets known as myosepta. The segmented architecture theory predicts; this phenomenon results in an architectural gear ratio, determined as longitudinal strain divided by fiber strain, greater than one and longitudinal velocity amplification. A red-to-white gearing ratio captures the combined effect of the longitudinal red muscle fiber and oblique white muscle fiber str
Lizards are a widespread group of squamate reptiles, with over 6,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic as it excludes Amphisbaenia. Lizards range in size from chameleons and geckos a few centimeters long to the 3 meter long Komodo dragon. Most lizards are quadrupedal. Others are legless, have long snake-like bodies; some such as the forest-dwelling Draco lizards are able to glide. They are territorial, the males fighting off other males and signalling with brightly colours, to attract mates and to intimidate rivals. Lizards are carnivorous being sit-and-wait predators. Lizards make use of a variety of antipredator adaptations, including venom, reflex bleeding, the ability to sacrifice and regrow their tails; the adult length of species within the suborder ranges from a few centimeters for chameleons such as Brookesia micra and geckos such as Sphaerodactylus ariasae to nearly 3 m in the case of the largest living varanid lizard, the Komodo dragon.
Most lizards are small animals. Lizards have four legs and external ears, though some are legless, while snakes lack these characteristics. Lizards and snakes share a movable quadrate bone, distinguishing them from the rhynchocephalians, which have more rigid diapsid skulls; some lizards such as chameleons have prehensile tails. As in other reptiles, the skin of lizards is covered in overlapping scales made of keratin; this reduces water loss through evaporation. This adaptation enables lizards to thrive in some of the driest deserts on earth; the skin is tough and leathery, is shed as the animal grows. Unlike snakes which shed the skin in a single piece, lizards slough their skin in several pieces; the scales may be modified into spines for display or protection, some species have bone osteoderms underneath the scales. The dentitions of lizards reflect their wide range of diets, including carnivorous, omnivorous, herbivorous and molluscivorous. Species have uniform teeth suited to their diet, but several species have variable teeth, such as cutting teeth in the front of the jaws and crushing teeth in the rear.
Most species are pleurodont, though chameleons are acrodont. The tongue can be extended outside the mouth, is long. In the beaded lizards and monitor lizards, the tongue is forked and used or to sense the environment, continually flicking out to sample the environment, back to transfer molecules to the vomeronasal organ responsible for chemosensation, analogous to but different from smell or taste. In geckos, the tongue is used to lick the eyes clean: they have no eyelids. Chameleons have long sticky tongues which can be extended to catch their insect prey. Three lineages, the geckos and chameleons, have modified the scales under their toes to form adhesive pads prominent in the first two groups; the pads are composed of millions of tiny setae which fit to the substrate to adhere using van der Waals forces. In addition, the toes of chameleons are divided into two opposed groups on each foot, enabling them to perch on branches as birds do. Aside from legless lizards, most lizards are quadrupedal and move using gaits with alternating movement of the right and left limbs with substantial body bending.
This body bending prevents significant respiration during movement, limiting their endurance, in a mechanism called Carrier's constraint. Several species can run bipedally, a few can prop themselves up on their hindlimbs and tail while stationary. Several small species such as those in the genus Draco can glide: some can attain a distance of 60 metres, losing 10 metres in height; some species, like chameleons, adhere to vertical surfaces including glass and ceilings. Some species, like the common basilisk, can run across water. Lizards make use of their senses of sight, touch and hearing like other vertebrates; the balance of these varies with the habitat of different species. Monitor lizards have acute vision and olfactory senses; some lizards make unusual use of their sense organs: chameleons can steer their eyes in different directions, sometimes providing non-overlapping fields of view, such as forwards and backwards at once. Lizards lack external ears, having instead a circular opening in which the tympanic membrane can be seen.
Many species rely on hearing for early warning of predators, flee at the slightest sound. As in snakes and many mammals, all lizards have a specialised olfactory system, the vomeronasal organ, used to detect pheromones. Monitor lizards transfer scent from the tip of their tongue to the organ; some lizards iguanas, have retained a photosensory organ on the top of their heads called the parietal eye, a basal feature present in the tuatara. This "eye" has only a rudimentary retina and lens and cannot form images, but is sensitive to changes in light and dark and can detect movemen
Georges Hébert was a pioneering physical educator in the French military who developed a system of physical education and training known as "la méthode naturelle", which combined the training of a wide variety of physical capacities with the training of courage and morality. Hébert was born in Paris. While an officer in the French Navy prior to the First World War, Hébert was stationed in the town of St. Pierre, on the island of Martinique in the Caribbean Sea. In 1902 the town fell victim to a catastrophic volcanic eruption. Hébert coordinated the rescue of some seven hundred people from this disaster; this experience had a profound effect on him, reinforced his belief that athletic skill must be combined with courage and altruism. He developed this ethos into his personal motto, "Être fort pour être utile". Hébert had travelled extensively throughout the world and was impressed by the physical development and movement skills of indigenous peoples in Africa and elsewhere: Their bodies were splendid, nimble, enduring and yet they had no other tutor in gymnastics but their lives in nature.
Upon his return to France, Hébert became a physical instructor for the French marines in Lorient, where he began to define the principles of his own system of physical education and to create apparatus and exercises to teach his "Natural Method". Beyond his observations of the natural movements of indigenous people, Hébert's method is a synthesis of various influences, including but not limited to: The work of his predecessor Francisco Amorós, who published in 1847 Nouveau Manuel Complet d'Education Physique, Gymnastique et Morale and which encompasses the full range of practical movement aptitudes The work of German Prussian gymnastics educator Friedrich Ludwig Jahn, which has probably influenced the early physical training of the United States Marine Corps The classical representations of the human body in Graeco-Roman statuary and by the ideals of the ancient Greek gymnasia The naturist lifestyle principles of his friend Dr. Paul Carton The influence of Georges Demenÿ, a French inventor, chronophotographer and gymnast who emphasized the progressiveness and the scalability of the trainingHebert's system rejected remedial gymnastics and the popular Swedish Method of physical culture, which seemed to him unable to develop the human body harmoniously and unable to prepare his students with the practical and moral demands of life.
In the same way, Hébert believed, by concentrating on competition and performance, competitive sport diverted physical education both from its physiological ends and its ability to foster sound moral values. For Georges Hébert, influenced by the teachings of philosopher and educationalist Jean-Jacques Rousseau, only the observation of nature could lead people to the true methods of physical development, he wrote: The final goal of physical education is to make strong beings. In the purely physical sense, the Natural Method promotes the qualities of organic resistance and speed, towards being able to walk, jump, move on all fours, to climb, to keep balance, to throw, defend yourself and to swim. In the "virile" or energetic sense, the system consists in having sufficient energy, courage and firmness. In the moral sense, education, by elevating the emotions, directs or maintains the moral drive in a useful and beneficial way; the true Natural Method, in its broadest sense, must be considered as the result of these three particular forces.
It resides not only in the muscles and the breath, but above all in the "energy", used, the will which directs it and the feeling which guides it. Hébert defined the guiding principles and fundamental rules of the Natural Method as: With regard to the development of virile qualities, this is obtained by the execution of certain difficult or dangerous exercises requiring the development of these various qualities, for example while seeking to control the fear of falling, of jumping, of rising, of plunging, of walking on an unstable surface, etc. Georges Hébert's teaching continued to expand between and during the two wars, becoming the standard system of French military physical education, he was an early advocate of the benefits of exercise for women. In his work "Muscle and Plastic Beauty", which appeared in 1921, Hébert criticized not only the fashion of corsetry but the physical inactivity imposed upon women by contemporary European society. By following the natural method of synthesized physical and moral development, he wrote, women could develop self-confidence, will-power and athletic ability just as well as their male counterparts.
Hébert wrote: A session is composed of exercises belonging to the ten fundamental groups: walking, jumping, quadrupedal movement, equilibrium, lifting and swimming. A training session consists of exercises in an outdoor environment a few hundred meters to several kilometers, during which, one walks, one runs, one jumps, one progresses quadrupedally, one climbs, one walks in unstable balance, one raises and one carries, one throws, one fights and one swims; this course can be carried out in two ways: the spontaneous way. Within an designed environment. All of the exercises can be carried out while progressing through this environment. A session can last from 20 to 60 minutes. Thus, Hébert was among the earliest proponents of le parcours, or obstacle course, f
Terrestrial locomotion has evolved as animals adapted from aquatic to terrestrial environments. Locomotion on land raises different problems than that in water, with reduced friction being replaced by the effects of gravity. There are three basic forms of locomotion found among terrestrial animals Legged – Moving by using appendages Limbless locomotion – moving without legs using the body itself as a propulsive structure. Rolling – rotating the body over the substrate Movement on appendages is the most common form of terrestrial locomotion, it is the basic form of locomotion of two major groups with many terrestrial members, the vertebrates and the arthropods. Important aspects of legged locomotion are posture, the number of legs, the functional structure of the leg and foot. There are many gaits, ways of moving the legs to locomote, such as walking, running, or jumping. Appendages can be used for movement in a lot of ways: the posture, the way the body is supported by the legs, is an important aspect.
There are three main ways in which vertebrates support themselves with their legs – sprawling, semi-erect, erect. Some animals may use different postures in different circumstances, depending on the posture's mechanical advantages. There is no detectable difference in energetic cost between stances; the "sprawling" posture is the most primitive, is the original limb posture from which the others evolved. The upper limbs are held horizontally, while the lower limbs are vertical, though upper limb angle may be increased in large animals; the body may drag along the ground, as in salamanders, or may be elevated, as in monitor lizards. This posture is associated with trotting gaits, the body flexes from side-to-side during movement to increase step length. All limbed reptiles and salamanders use this posture, as does the platypus and several species of frogs that walk. Unusual examples can be found among amphibious fish, such as the mudskipper, which drag themselves across land on their sturdy fins.
Among the invertebrates, most arthropods – which includes the most diverse group of animals, the insects – have a stance best described as sprawling. There is anecdotal evidence that some octopus species can drag themselves across land a short distance by hauling their body along by their tentacles – there may be video evidence of this; the semi-erect posture is more interpreted as an elevated sprawling posture. This mode of locomotion is found in large lizards such as monitor lizards and tegus. Mammals and birds have a erect posture, though each evolved it independently. In these groups the legs are placed beneath the body; this is linked with the evolution of endothermy, as it avoids Carrier's constraint and thus allows prolonged periods of activity. The erect stance is not the "most-evolved" stance. For example, the mesozoic prehistoric crocodilian Erpetosuchus is believed to have had a erect stance and been terrestrial; the number of locomotory appendages varies much between animals, sometimes the same animal may use different numbers of its legs in different circumstances.
The best contender for unipedal movement is the springtail, which while hexapedal, hurls itself away from danger using its furcula, a tail-like forked rod that can be unfurled from the underside of its body. A number of species stand on two legs, that is, they are bipedal; the group, bipedal is the birds, which have either an alternating or a hopping gait. There are a number of bipedal mammals. Most of these move by hopping – including the macropods such as kangaroos and various jumping rodents. Only a few mammals such as humans and the ground pangolin show an alternating bipedal gait. Cockroaches and some lizards may run on their two hind legs. With the exception of the birds, terrestrial vertebrate groups with legs are quadrupedal – the mammals and the amphibians move on four legs. There are many quadrupedal gaits; the most diverse group of animals on earth, the insects, are included in a larger taxon known as hexapods, most of which are hexapedal and standing on six legs. Exceptions among the insects include praying mantises and water scorpions, which are quadrupeds with their front two legs modified for grasping, some butterflies such as the Lycaenidae which use only four legs, some kinds of insect larvae that may have no legs, or additional prolegs.
Spiders and many of their relatives move on eight legs – they are octopedal. However, some creatures move on many more legs. Terrestrial crustaceans may have a fair number – woodlice having fourteen legs; as mentioned, some insect larvae such as caterpillars and sawfly larvae have up to five or nine additional fleshy prolegs in addition to the six legs normal for insects. Some species of invertebrate have more legs, the unusual velvet worm having stubby legs under the length of its body, with around several dozen pairs of legs. Centipedes have one pair of legs per body segment, with around 50 legs, but some species have over 200; the terrestrial animals with the most legs are the millipedes. They have two pairs of legs per body segment, with common species having between 80 and 400 legs overall – with the rare species Illacme plenipe