Animals

Animals area major group of multicellular, eukaryotic organisms of the kingdom Animalia. Their body plan eventually becomes fixed as they develop, althoughsome undergo a process of metamorphosis later on in their life. Most animalsare motile, meaning they can move spontaneously and independently. Animals arealso heterotrophs, meaning they must ingest other organisms for sustenance.
Most knownanimal phyla appeared in the fossil record as marine species during theCambrian explosion, about 542 million years ago.

Scientistshave divided animalss into two main groups: Vertebrates(animals with a backbone) and Invertebrates(animals without a backbone)
Animalswith backbones can be divided into five more groups: Mammals,Birds, Fish, Reptile, Amphibians

The wordmammal comes from the Latin mamma, meaning breast, because female mammalsproduce milk to nurse their babies. Nearly all mammals give birth to liveyoung, and all are warm-blooded, maintaining a near-constant body temperatureregardless of environmental conditions. They are vertebrates and use lungs tobreathe air and are the only animals that grow hair. Mammals are the onlyanimals with hair or fur that protects it agains the elements - heat, cold,wind, rain, bacteria, and even infection. Mammalsprobably appeared on Earth some 200 million years ago.

There areabout 10,000 bird species, found in every shape and size, from the tinyhummingbird to the majestic condor. They occur in all the colors of the rainbowand are loved for their musical songs. Some birds can even mimic human speech.All birds lay eggs to incubate their young, and all have feathers—a trait noother animals can claim. Birds also have wings, evolutionary adaptations ofancient forelimbs. Most use their wings for flight, but others, like ostrichesand penguins, are grounded. 

Fish areancient water-dwelling creatures found all over the globe. They are the world'soldest vertebrates, with the first fish appearing some 500 million years ago.Today more than 24,000 species populate the waters of the world. Fish arevertebrates and breathe using gills, which draw oxygen from the water and intothe bloodstream. Most have bony skeletons, though sharks and rays haveskeletons made of cartilage. Nearly all reproduce by laying eggs. 

Amphibianslead double lives—one in water and one on land. Many begin life with gills,then develop lungs as they age. They are vertebrate animals that include frogs,toads, salamanders, and newts as well as odd, wormlike caecilians(seh-SILL-yuns). They are cold-blooded, using the environment to regulate theirbody temperature. Early amphibians were the first animals to leave the sea andventure onto land, forming a crucial link from fish to terrestrial reptiles. 

Reptileshave been around for 300 million years, and during the age of the dinosaurs,they ruled the Earth. Those days are long gone, and those giants have vanished,but some 6,500 species of reptiles still thrive today. Crocodiles, snakes,lizards, and turtles are all reptiles. Most reptiles live on land, and most layeggs. They are vertebrates, and, unlike any other animals, are covered inscales. They are cold-blooded, and regulate their body temperature by seekingor avoiding the sun's heat.

Etymology of animals:
The word"animal" comes from the Latin word animale, neuter of animalis, andis derived from anima, meaning vital breath or soul.

Characteristics of animals:
Animalshave several characteristics that set them apart from other living things.Animals are eukaryotic and usually multicellular, which separates them frombacteria and most protists. They are heterotrophic, generally digesting food inan internal chamber, which separates them from plants and algae. They are alsodistinguished from plants, algae, and fungi by lacking cell walls. In mostanimals, embryos pass through a blastula stage, which is a characteristicexclusive to animals.

Structure of animals:
Animalshave bodies differentiated into separate tissues. These include muscles, whichare able to contract and control locomotion, and nerve tissue, which sends andprocesses signals. All animals have eukaryotic cells, surrounded by acharacteristic extracellular matrix composed of collagen and elasticglycoproteins. This may be calcified to form structures like shells, bones, andspicules. During development it forms a relatively flexible framework upon whichcells can move about and be reorganized, making complex structures possible.

Food andenergy sourcing of animals:
Predation is a biological interaction where apredator feeds on its prey. Predators may or may not kill their prey prior tofeeding on them, but the act of predation always results in the death of theprey. Selective pressures imposed on one another has lead to an evolutionaryarms race between prey and predator, resulting in various antipredatoradaptations.
Mostanimals feed indirectly from the energy of sunlight. Plants use this energy toconvert sunlight into simple sugars using a process known as photosynthesis.When animals eat these plants (or eat other animals which have eaten plants),the sugars produced by the plant are used by the animal. They are either useddirectly to help the animal grow, or broken down, releasing stored solarenergy, and giving the animal the energy required for motion.