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                          ANIMALS

Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually 0are motile, meaning they can move spontaneously and independently. All animals are also heterotrophs, meaning they must ingest other organisms or their products for sustenance.

Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago.

AMPHIBIANS

Amphibians are members of the class Amphibia, whose living forms include frogs, toads, salamanders, newts and caecilians. They are cold-blooded, tetrapod vertebrates. Most have four limbs and live in fresh water or on land but the caecilians, though included in the group, live in burrows in damp soil and are limbless. Amphibians typically have a larval stage that lives in water but there are many different behavioural adaptations that have developed among species to bypass this necessity. The young undergo metamorphosis from a larval form with gills to an adult air-breathing form with lungs. Amphibians also use their skin for respiration and some small terrestrial salamanders rely entirely on this and have no lungs.

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), Caudata/Urodela (the salamanders), and Gymnophiona/Apoda (the caecilians). The total number of known amphibian species is approximately 7,000, of which nearly 90% are frogs. They are superficially similar to reptiles but, along with mammals and birds, reptiles are amniotes and no longer require water bodies in which to breed. With their often complex reproductive needs and permeable skins, amphibians are often ecological indicators and in recent decades there has been a dramatic decline in amphibian populations of many species around the globe. The smallest vertebrate in the world is the New Guinea frog, Paedophryne amauensis.[1] The largest amphibian is the Chinese Giant Salamander, Andrias davidianus.[2] The study of amphibians is called batrachology while the study of both reptiles and amphibians is called herpetology.

MAMMALS

Mammals are members of class Mammalia (play /məˈmeɪli.ə/), air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young. Most mammals also possess sweat glands and specialised teeth. The largest group of mammals, the placentals, have a placenta which feeds the offspring during gestation. The mammalian brain, with its characteristic neocortex, regulates endothermic and circulatory systems, the latter featuring red blood cells lacking nuclei and a four-chambered heart. Mammals range in size from the 30–40 millimeter (1- to 1.5-inch) bumblebee bat to the 33-meter (108-foot) blue whale.

The word “mammal” is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma (“teat, pap”). All female mammals nurse their young with milk, which is secreted from special glands, the mammary glands. According to Mammal Species of the World, 5,702 species were known in 2005. These were grouped in 1,229 genera, 153 families and 29 orders.[1] In 2008 the IUCN completed a five-year, 17,000-scientist Global Mammal Assessment for its IUCN Red List, which counted 5,488 accepted species at the end of that period.[2] In some classifications, the mammals are divided into two subclasses (not counting fossils): the Prototheria (order of Monotremata) and the Theria, the latter composed of the infraclasses Metatheria and Eutheria. The marsupials comprise the crown group of the Metatheria and therefore include all living metatherians as well as many extinct ones; the placentals likewise constitute the crown group of the Eutheria.

Except for the five species of monotremes (which lay eggs), all living mammals give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders, in descending order, are Rodentia (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals), Chiroptera (bats), and Soricomorpha (shrews, moles and solenodons). The next three largest orders, depending on the classification scheme used, are the primates, to which the human species belongs, the Cetartiodactyla (including the even-toed hoofed mammals and the whales), and the Carnivora (cats, dogs, weasels, bears, seals, and their relatives).[1] While the classification of mammals at the family level has been relatively stable, different treatments at higher levels—subclass, infraclass, and order—appear in contemporaneous literature, especially for the marsupials. Much recent change has reflected the results of cladistic analysis and molecular genetics. Results from molecular genetics, for example, have led to the adoption of new groups such as the Afrotheria and the abandonment of traditional groups such as the Insectivora.

The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that also included Dimetrodon. At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today’s reptiles and birds. Preceded by many diverse groups of non-mammalian synapsids (sometimes referred to as mammal-like reptiles), the first mammals appeared in the early Mesozoic era. The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era.

REPTILES

Traditionally, Reptiles are members of the class Reptilia comprising the amniotes that are neither birds nor mammals.[1] (The amniotes are the vertebrates with eggs featuring an amnion, a double membrane that permits the embryo to breath effectively on land.) Living reptiles can be distinguished from other tetrapods in that they are cold-blooded as well as bearing scutes or scales.

Reptiles originated around 320-310 million years ago during the Carboniferous period, having evolved from advanced reptile-like amphibians that became increasingly adapted to life on dry land. There are many extinct groups, including dinosaurs, pterosaurs, and aquatic groups such as the ichthyosaurs. Modern reptiles inhabit every continent with the exception of Antarctica. Several living sub-groups are recognized:

Although they have scutes on their feet and lay eggs, birds have historically been excluded from the reptiles, in part because they are warm-blooded. They therefore do not appear on the list above. However, as some reptiles are more closely related to birds than they are to other reptiles—crocodiles are more closely related to birds than they are to lizards—cladistic writers who prefer a more unified (monophyletic) grouping sometimes also include the birds with over 10,000 species, see Sauropsida.

Reptiles are tetrapod vertebrates, either having four limbs or, like snakes, being descended from four-limbed ancestors. Unlike amphibians, reptiles do not have an aquatic larval stage. Most reptiles are oviparous (egg-laying), although certain species of squamates retain the eggs until hatching and a few are viviparous (give birth to live young). As amniotes, reptile eggs are surrounded by membranes for protection and transport that adapt them to reproduction on dry land. Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings. Extant reptiles range in size from a tiny gecko, Sphaerodactylus ariasae, which can grow up to 17 mm (0.7 in) to the saltwater crocodile, Crocodylus porosus, which may reach 6 m (19.7 ft) in length and weigh over 1,000 kg (2,200 lb).

Traditionally, the reptiles are one of the three classes of amniotes, the others being the mammals and the birds. By the middle of the 20th century, it had become common to split the reptiles into four groups based on their skull openings. However, as reptiles in the traditional sense are the ancestors of both birds and mammals, many taxonomists have more recently adopted a cladistic approach, preferring to group animals based exclusively on shared ancestry rather than on shared features; this orientation has led some to abandon reptiles as a systematic unit or redefine the reptiles in a manner that includes birds.

The historically combined study of reptiles and amphibians is called herpetology.

FISH

A fish is any member of a paraphyletic group of organisms that consist of all gill-bearing aquatic craniate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups. Most fish are ectothermic (“cold-blooded”), allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature.[1][2] Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon) to the abyssal and even hadal depths of the deepest oceans (e.g., gulpers and anglerfish). At 32,000 species, fish exhibit greater species diversity than any other group of vertebrates.[3]

Fish are an important resource worldwide, especially as food. Commercial and subsistence fishers hunt fish in wild fisheries (see fishing) or farm them in ponds or in cages in the ocean (see aquaculture). They are also caught by recreational fishers, kept as pets, raised by fishkeepers, and exhibited in public aquaria. Fish have had a role in culture through the ages, serving as deities, religious symbols, and as the subjects of art, books and movies.

Because the term “fish” is defined negatively, and excludes the tetrapods (i.e., the amphibians, reptiles, birds and mammals) which descend from within the same ancestry, it is paraphyletic, and is not considered a proper grouping in systematic biology. The traditional term pisces (also ichthyes) is considered a typological, but not a phylogenetic classification.

The earliest organisms that can be classified as fish were soft-bodied chordates that first appeared during the Cambrian period. Although they lacked a true spine, they possessed notochords which allowed them to be more agile than their invertebrate counterparts. Fish would continue to evolve through the Paleozoic era, diversifying into a wide variety of forms. Many fish of the Paleozoic developed external armor that protected them from predators. The first fish with jaws appeared in the Silurian period, after which many (such as sharks) became formidable marine predators rather than just the prey of arthropods.

BIRDS

Birds (class Aves) are feathered, winged, bipedal, endothermic (warm-blooded), egg-laying, vertebrate animals. With around 10,000 living species, they are the most speciose class of tetrapod vertebrates. All present species belong to the subclass Neornithes, and inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from the 5 cm (2 in) Bee Hummingbird to the 2.75 m (9 ft) Ostrich. The fossil record indicates that birds emerged within theropod dinosaurs during the Jurassic period, around 160 million years (Ma) ago. Paleontologists regard birds as the only clade of dinosaurs to have survived the Cretaceous–Paleogene extinction event 65.5 Ma ago.

Modern birds are characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. All living species of birds have wings—the now extinct flightless moa of New Zealand being the only exception. Wings are evolved forelimbs, and most bird species can fly. Flightless birds include ratites, penguins, and a number of diverse endemic island species. Birds also have unique digestive and respiratory systems that are highly adapted for flight. Some birds, especially corvids and parrots, are among the most intelligent animal species; a number of bird species have been observed manufacturing and using tools, and many social species exhibit cultural transmission of knowledge across generations.

Many species undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social; they communicate using visual signals and through calls and songs, and participate in social behaviours, including cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially monogamous, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have polygynous (“many females”) or, rarely, polyandrous (“many males”) breeding systems. Eggs are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching.

Many species are of economic importance, mostly as sources of food acquired through hunting or farming. Some species, particularly songbirds and parrots, are popular as pets. Other uses include the harvesting of guano (droppings) for use as a fertiliser. Birds figure prominently in all aspects of human culture from religion to poetry to popular music. About 120–130 species have become extinct as a result of human activity since the 17th century, and hundreds more before then. Currently about 1,200 species of birds are threatened with extinction by human activities, though efforts are underway to protect them.

http://en.wikipedia.org/wiki/Animal