Dictionary Definition
animal adj
1 of the appetites and passions of the body;
"animal instincts"; "carnal knowledge"; "fleshly desire"; "a
sensual delight in eating"; "music is the only sensual pleasure
without vice" [syn: animal(a),
carnal, fleshly, sensual]
2 of the nature of or characteristic of or
derived from an animal or animals; "the animal kingdom"; "animal
instincts"; "animal fats" [ant: vegetable, mineral] n : a living organism
characterized by voluntary movement [syn: animate
being, beast, brute, creature, fauna]
User Contributed Dictionary
English
Pronunciation
- , /ˈænəməl/, /"
Extensive Definition
Animals are a major group of
multicellular,
eukaryotic organisms of the kingdom
Animalia or Metazoa. Their body plan
eventually becomes fixed as they develop, although some undergo a
process of metamorphosis
later on in their life. Most animals are motile - they can move
spontaneously and independently. Animals are heterotrophs - they are
dependent on other organisms (e.g. plants) for sustenance.
Most known animal phyla
appeared in the fossil record as marine species during the Cambrian
explosion, about 542 million years ago.
Etymology
The word "animal" comes from the Latin word animale, neuter of animalis, and is derived from anima, meaning vital breath or soul. In everyday colloquial usage, the word usually refers to non-human animals. The biological definition of the word refers to all members of the Kingdom Animalia. Therefore, when the word "animal" is used in a biological context, humans are included.Characteristics
Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and usually multicellular (although see Myxozoa), which separates them from bacteria and most protists. They are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae. They are also distinguished from plants, algae, and fungi by lacking cell walls. All animals are motile, if only at certain life stages. In most animals, embryos pass through a blastula stage, which is a characteristic exclusive to animals.Structure
With a few exceptions, most
notably the sponges
(Phylum Porifera), animals have bodies differentiated into separate
tissues.
These include muscles,
which are able to contract and control locomotion, and nerve
tissue, which sends and processes signals. There is also
typically an internal digestive chamber, with one or
two openings. Animals with this sort of organization are called
metazoans, or eumetazoans when the former
is used for animals in general.
All animals have eukaryotic cells, surrounded
by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This may be
calcified to form structures like shells,
bones, and spicules. During development it
forms a relatively flexible framework upon which cells can move
about and be reorganized, making complex structures possible. In
contrast, other multicellular organisms like plants and fungi have
cells held in place by cell walls, and so develop by progressive
growth. Also, unique to animal cells are the following
intercellular junctions: tight
junctions, gap
junctions, and desmosomes.
Reproduction and development
Nearly all animals undergo
some form of sexual reproduction. Adults are diploid or polyploid. They have a few
specialized reproductive cells, which undergo meiosis to produce smaller
motile spermatozoa
or larger non-motile ova.
These fuse to form zygotes, which develop into new
individuals.
Many animals are also capable
of asexual
reproduction. This may take place through parthenogenesis, where
fertile eggs are produced without mating, or in some cases through
fragmentation.
A zygote initially develops into a
hollow sphere, called a blastula, which undergoes
rearrangement and differentiation. In sponges, blastula larvae swim
to a new location and 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
mesoderm also develops
between them. These germ layers then differentiate to form tissues
and organs.
Most animals grow by
indirectly using the energy of sunlight. Plants use this
energy to convert
sunlight into simple sugars using a process known as
photosynthesis.
Starting with the molecules carbon
dioxide (CO2) and water (H2O), photosynthesis
converts the energy of sunlight into chemical energy stored in the
bonds of glucose
(C6H12O6) and releases oxygen (O2). These sugars are
then used as the building blocks which allow the plant to grow.
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 used directly to help the animal grow, or
broken down, releasing stored solar energy, and giving the animal
the energy required for motion. This process is known as glycolysis.
Animals who live close to
hydrothermal
vents and cold seeps on
the ocean floor are not dependent on the energy of sunlight.
Instead, chemosynthetic archaea and eubacteria form the base of
the food chain.
Origin and fossil record
Animals are generally considered to have evolved from a flagellated eukaryote. Their closest known living relatives are the choanoflagellates, collared flagellates that have a morphology similar to the choanocytes of certain sponges. Molecular studies place animals in a supergroup called the opisthokonts, which also include the choanoflagellates, fungi and a few small parasitic protists. The name comes from the posterior location of the flagellum in motile cells, such as most animal spermatozoa, whereas other eukaryotes tend to have anterior flagella.The first fossils that might
represent animals appear towards the end of the Precambrian,
around 610 million years ago, and are known as the Ediacaran or
Vendian biota. These are difficult to relate to later fossils,
however. Some may represent precursors of modern phyla, but they
may be separate groups, and it is possible they are not really
animals at all. Aside from them, most known animal phyla make a
more or less simultaneous appearance during the Cambrian period,
about 542 million years ago. It is still disputed whether this
event, called the Cambrian
explosion, represents a rapid divergence between different
groups or a change in conditions that made fossilization possible.
However some paleontologists and geologists would suggest that
animals appeared much earlier than previously thought, possibly
even as early as 1 billion years ago. Trace fossils such as tracks
and burrows found in Tonian era strata in
India indicate the presence of triploblastic worm like
metazoans roughly as
large (about 5mm wide) and complex as earthworms. In addition
during the beginning of the Tonian period around 1 billion years
ago (roughly the same time that the trace fossils previously
discussed in this article date back to) there was a decrease in
Stromatolite
diversity which may indicate the appearance of grazing animals
during this time as Stromatolites also increased in diversity
shortly after the end-Ordovician and end-Permian rendered large
amounts of grazing marine animals extinct and decreased shortly
after their populations recovered. However some other scientists
doubt that these fossils are authentic and have suggested these
trace fossils are just the result of natural processes such as
erosion.
Groups of animals
The sponges (Porifera) were long thought to have diverged from other animals early. As mentioned above, they lack the complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct tissues. Sponges are sessile and typically feed by drawing in water through pores. Archaeocyatha, which have fused skeletons, may represent sponges or a separate phylum. However, a phylogenomic study in 2008 of 150 genes in 21 genera revealed that it is the Ctenophora or comb jellies which are the basal lineage of animals, at least among those 21 phyla. The authors speculate that sponges—or at least those lines of sponges they investigated—are not so primitive, but may instead be secondarily simplified.Among the other phyla, the
Ctenophora and the Cnidaria, which
includes sea anemones,
corals, and jellyfish, are radially
symmetric and have digestive chambers with a single opening, which
serves as both the mouth and the anus. Both have distinct tissues,
but they are not organized into organs.
There are only two main germ layers, the ectoderm and endoderm,
with only scattered cells between them. As such, these animals are
sometimes called diploblastic. The tiny
Placozoans
are similar, but they do not have a permanent digestive
chamber.
The remaining animals form a
monophyletic group
called the Bilateria. For
the most part, they are bilaterally symmetric, and often have a
specialized head with feeding and sensory organs. The body is
triploblastic,
i.e. all three germ layers are well-developed, and tissues form
distinct organs. The digestive chamber has two openings, a mouth
and an anus, and there is also an internal body cavity called a
coelom or pseudocoelom.
There are exceptions to each of these characteristics, however -
for instance adult echinoderms are radially
symmetric, and certain parasitic worms have extremely simplified
body structures.
Genetic studies have
considerably changed our understanding of the relationships within
the Bilateria. Most appear to belong to two major lineages: the
Deuterostomes
and Protostomes,
which includes the Ecdysozoa,
Platyzoa,
and Lophotrochozoa.
In addition, there are a few small groups of bilaterians with
relatively similar structure that appear to have diverged before
these major groups. These include the Acoelomorpha,
Rhombozoa, and
Orthonectida.
The Myxozoa,
single-celled parasites that were originally considered Protozoa,
are now believed to have developed from the Bilateria as
well.
Deuterostomes
Deuterostomes differ from the other Bilateria, called protostomes, in several ways. In both cases there is a complete digestive tract. However, in protostomes the initial opening (the archenteron) develops into the mouth, and an anus forms separately. In deuterostomes this is reversed. In most protostomes, cells simply fill in the interior of the gastrula to form the mesoderm, called schizocoelous development, but in deuterostomes it forms through invagination of the endoderm, called enterocoelic pouching. Deuterostomes also have a dorsal, rather than a ventral, nerve chord and their embryos undergo different cleavage.All this suggests the
deuterostomes and protostomes are separate, monophyletic lineages.
The main phyla of deuterostomes are the Echinodermata
and Chordata. The
former are radially symmetric and exclusively marine, such as
starfish, sea urchins,
and sea
cucumbers. The latter are dominated by the vertebrates, animals with
backbones. These include fish, amphibians, reptiles, birds, and mammals.
In addition to these, the
deuterostomes also include the Hemichordata
or acorn worms. Although they are not especially prominent today,
the important fossil graptolites may belong to
this group.
The Chaetognatha
or arrow worms may also be deuterostomes, but more recent studies
suggest protostome affinities.
Ecdysozoa
The Ecdysozoa are protostomes, named after the common trait of growth by moulting or ecdysis. The largest animal phylum belongs here, the Arthropoda, including insects, spiders, crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits.The ecdysozoans also include
the Nematoda or
roundworms, the second largest animal phylum. Roundworms are
typically microscopic, and occur in nearly every environment where
there is water. A number are important parasites. Smaller phyla
related to them are the Nematomorpha
or horsehair worms, and the Kinorhyncha,
Priapulida, and
Loricifera.
These groups have a reduced coelom, called a
pseudocoelom.
The remaining two groups of
protostomes are sometimes grouped together as the Spiralia, since in
both embryos develop with spiral cleavage.
Platyzoa
The Platyzoa include the phylum Platyhelminthes, the flatworms. These were originally considered some of the most primitive Bilateria, but it now appears they developed from more complex ancestors.A number of parasites are included in this
group, such as the flukes
and tapeworms.
Flatworms are
acoelomates, lacking a body cavity, as are their closest
relatives, the microscopic Gastrotricha.
The other platyzoan phyla are
mostly microscopic and pseudocoelomate.
The most prominent are the Rotifera or
rotifers, which are common in aqueous environments. They also
include the Acanthocephala
or spiny-headed worms, the Gnathostomulida,
Micrognathozoa,
and possibly the Cycliophora.
These groups share the presence of complex jaws, from which they
are called the Gnathifera.
Lophotrochozoa
The Lophotrochozoa include two of the most successful animal phyla, the Mollusca and Annelida. The former includes animals such as snails, clams, and squids, and the latter comprises the segmented worms, such as earthworms and leeches. These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods, because they are both segmented. Now this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.The Lophotrochozoa also
include the Nemertea or ribbon
worms, the Sipuncula, and
several phyla that have a fan of cilia around the mouth, called a
lophophore. These
were traditionally grouped together as the lophophorates. but it
now appears they are paraphyletic, some closer
to the Nemertea and some to the Mollusca and Annelida. They include
the Brachiopoda or
lamp shells, which are prominent in the fossil record, the Entoprocta, the
Phoronida, and
possibly the Bryozoa or moss
animals.
Model organisms
Because of the great diversity found in animals, it is more economical for scientists to study a small number of chosen species so that connections can be drawn from their work and conclusions extrapolated about how animals function in general. Because they are easy to keep and breed, the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans have long been the most intensively studied metazoan model organisms, and were among the first lifeforms to be genetically sequenced. This was facilitated by the severely reduced state of their genomes, but the double-edged sword here is that with many genes, introns and linkages lost, these ecdysozoans can teach us little about the origins of animals in general. The extent of this type of evolution within the superphylum will be revealed by the crustacean, annelid, and molluscan genome projects currently in progress. Analysis of the starlet sea anemone genome has emphasised the importance of sponges, placozoans, and choanoflagellates, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to the Eumetazoa.An analysis of the
homoscleromorph sponge Oscarella carmela also suggests that the
last common ancestor of sponges and the eumetazoan animals was more
complex than previously assumed.
Other model organisms
belonging to the animal kingdom include the mouse (Mus
musculus) and zebrafish (Danio
rerio}.
History of classification
Aristotle divided the living world between animals and plants, and this was followed by Carolus Linnaeus (Carl von Linné), in the first hierarchical classification. Since then biologists have begun emphasizing evolutionary relationships, and so these groups have been restricted somewhat. For instance, microscopic protozoa were originally considered animals because they move, but are now treated separately.In Linnaeus's
original scheme, the animals were one of three kingdoms, divided
into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then
the last four have all been subsumed into a single phylum, the
Chordata, whereas the
various other forms have been separated out. The above lists
represent our current understanding of the group, though there is
some variation from source to source.
See also
Notes
References
- Klaus Nielsen. Animal Evolution: Interrelationships of the Living Phyla (2nd edition). Oxford Univ. Press, 2001.
- Knut Schmidt-Nielsen. Animal Physiology: Adaptation and Environment. (5th edition). Cambridge Univ. Press, 1997.
External links
- Tree of Life Project
- Animal Diversity Web - University of Michigan's database of animals, showing taxonomic classification, images, and other information.
- ARKive - multimedia database of worldwide endangered/protected species and common species of UK.
- Scientific American Magazine (December 2005 Issue) - Getting a Leg Up on Land About the evolution of four-limbed animals from fish.
animal in Afrikaans:
Animalia
animal in Tosk Albanian:
Tiere
animal in Arabic:
حيوان
animal in Aragonese:
Animal
animal in Franco-Provençal:
Animâl
animal in Asturian:
Animal
animal in Guarani:
Mymba
animal in Aymara:
Uywa
animal in Bambara:
Bagan
animal in Min Nan:
Tōng-bu̍t
animal in Bosnian:
Životinjska bića
animal in Breton:
Loen
animal in Bulgarian:
Животни
animal in Catalan:
Animal
animal in Czech:
Živočichové
animal in Welsh:
Anifail
animal in Danish:
Dyr
animal in Pennsylvania
German: Gedier
animal in German:
Tier
animal in Navajo:
Naaldloshii
animal in Estonian:
Loomad
animal in Modern Greek
(1453-): Ζώο
animal in Erzya:
Ракшат
animal in Spanish:
Animalia
animal in Esperanto:
Animalo
animal in Basque:
Animalia
animal in Persian:
جانوران
animal in Faroese:
Dýr
animal in French:
Animal
animal in Western Frisian:
Dier
animal in Irish:
Ainmhí
animal in Gan Chinese:
動物
animal in Scottish Gaelic:
Beathach
animal in Galician:
Animalia
animal in Korean:
동물
animal in Croatian:
Životinje
animal in Ido:
Animalo
animal in Indonesian:
Hewan
animal in Interlingua
(International Auxiliary Language Association): Animal
animal in Inuktitut:
ᓂᕐᔪᑦ/nirjut
animal in Icelandic:
Dýr
animal in Italian:
Animalia
animal in Hebrew: בעלי
חיים
animal in Pampanga:
Animal
animal in Kannada:
ಪ್ರಾಣಿ
animal in Cornish:
Enyval
animal in Haitian:
Zannimo
animal in Kurdish:
Ajal
animal in Latin:
Animalia
animal in Latvian:
Dzīvnieki
animal in Luxembourgish:
Déiereräich
animal in Lithuanian:
Gyvūnai
animal in Limburgan:
Diere
animal in Lojban:
danlu
animal in Hungarian:
Állatok
animal in Macedonian:
Животни
animal in Malayalam:
ജീവി
animal in Malay
(macrolanguage): Haiwan
animal in Mongolian:
Амьтан
nah:Yōlcatl
animal in Fijian:
Manumanu
animal in Dutch: Dieren
(rijk)
animal in Japanese:
動物
animal in Norwegian:
Dyr
animal in Norwegian Nynorsk:
Dyr
animal in Narom:
Animâ
animal in Novial:
Animalia
animal in Occitan (post
1500): Animalia
animal in Oromo:
Binensotta
animal in Low German:
Beester
animal in Polish:
Zwierzęta
animal in Portuguese:
Animalia
animal in Kölsch:
Dier
animal in Romanian: Regn
Animalia
animal in Quechua:
Uywa
animal in Russian:
Животные
animal in Albanian:
Kafsha
animal in Sicilian:
Armali
animal in Simple English:
Animal
animal in Slovak:
Živočíchy
animal in Slovenian:
Živali
animal in Serbian:
Животиње
animal in Serbo-Croatian:
Životinje
animal in Sundanese:
Sato
animal in Finnish:
Eläinkunta
animal in Swedish:
Djur
animal in Tagalog:
Hayop
animal in Tamil:
விலங்கு
animal in Tarantino: Regnum
Animalia
animal in Thai:
สัตว์
animal in Vietnamese: Động
vật
animal in Tonga (Tonga
Islands): Monumanu
animal in Cheyenne:
Hova
animal in Turkish:
Hayvanlar
animal in Ukrainian:
Тварини
animal in Vlaams: Bêesten
(ryk)
animal in Yiddish: בעלי
חיים
animal in Contenese:
動物
animal in Zeeuws:
Beêsten
animal in Samogitian:
Gīvūnā
animal in Chinese:
动物
Synonyms, Antonyms and Related Words
Adamic, Angora goat, Arctic fox,
Belgian hare, Caffre cat, Circean, Draconian, Goth, Gothic, Indian buffalo, Kodiak
bear, Neanderthal,
Tartarean, Virginia
deer, aardvark,
aardwolf, alpaca, ammonite, animalian, animalic, animalistic, ankylosaur, anteater, antelope, antelope chipmunk,
anthropophagite,
anthropophagous,
aoudad, apar, aphrodisiomaniacal,
archaeohippus,
archaeotherium,
archelon, armadillo, arthrodiran, ass, atrocious, aurochs, badger, bandicoot, barbarian, barbaric, barbarous, bassarisk, bat, bawdy, bear, beast, beastlike, beastly, beaver, being, bestial, bettong, binturong, bison, black bear, black buck,
black cat, black fox, black sheep, bloodthirsty, bloody, bloody-minded, blue fox,
bobcat, bodily, bothriolepis, brachiosaur, brontops, brontothere, brown bear,
brush deer, brush wolf, brutal, brutalized, brute, brutelike, brutish, buffalo, buffalo wolf, burro, burro deer, cachalot, camel, camelopard, cannibal, cannibalistic, capybara, carabao, caribou, carnal, carpincho, cat, cat-a-mountain, catamount, cattalo, cavy, chamois, cheetah, chevrotain, chinchilla, chipmunk, cinnamon bear,
clitoromaniacal,
coarse, coccostean, coelodont, compsognathus, concupiscent, coon, coon cat, coryphodon, cotton mouse,
cotton rat, cotylosaur, cougar, cow, coyote, coypu, creature, creeping thing,
creodont, critter, crossopterygian,
crude, cruel, cruel-hearted, cur, cynodictis, deer, deer tiger, demoniac, demoniacal, destroyer, devilish, diabolic, diatryma, dimetrodon, dingo, dinichthyid, dinothere, diplodocus, dirty, dog, donkey, dormouse, dromedary, duck-billed
dinosaur, dumb, dumb
animal, dumb friend, earthy, echidna, edaphosaurid, eland, elasmosaur, elephant, elk, ermine, erotic, eroticomaniacal,
erotomaniacal,
eryopsid, eurypterid, eyra, fallen, fallow deer, fell, feral, ferine, ferocious, ferret, field mouse, fiendish, fiendlike, fierce, fisher, fitch, fleshly, flying phalanger,
foumart, fox, fox squirrel, gazelle, gemsbok, genet, giant sloth, giraffe, glutton, glyptodont, gnu, gnu goat, goat, goat antelope, goatish, gopher, grizzly bear, gross, ground squirrel, groundhog, guanaco, guinea pig, gynecomaniacal, hadrosaur, hamster, hare, harnessed antelope, hartebeest, hedgehog, hellish, hippopotamus, hog, horny, horse, hot, hound, hyena, hyrax, hysteromaniacal,
ibex, ill-bred, impolite, infernal, inhuman, inhumane, insect, instinctive, instinctual, ithyphallic, jackal, jackass, jackrabbit, jaguar, jaguarundi, jerboa, jerboa kangaroo, kaama, kangaroo, kangaroo mouse,
kangaroo rat, karakul,
kinkajou, kit fox,
koala, lapin, lapsed, lascivious, lecherous, lemming, leopard, leopard cat, lewd, libidinous, lickerish, lion, living being, living thing,
llama, lubricious, lustful, lynx, mammal, mammoth, man-eater, mara, marmot, marten, mastodon, material, materialistic, meerkat, mindless, mink, mole, mongoose, mongrel, monster, moose, mouflon, mountain goat, mountain
lion, mountain sheep, mouse, mule, mule deer, muntjac, murderous, musk deer, musk
hog, musk-ox, muskrat,
musquash, nihilist, nilgai, noncivilized, nonrational, nonspiritual, nutria, nymphomaniacal, obscene, ocelot, okapi, onager, oont, opossum, organism, orgiastic, otter, ounce, outlandish, ox, pack rat, painter, palaeodictyopteron,
palaeoniscid,
palaeophis, palaeosaur, palaeospondylus,
panda, pangolin, panther, peccary, peludo, pelycosaur, phalanger, physical, phytosaur, pig, pine mouse, platypus, plesiosaur, pocket gopher,
pocket mouse, pocket rat, polar bear, polar fox, polecat, porcupine, possum, postlapsarian, pouched
rat, poyou, prairie dog,
prairie wolf, priapic,
primitive, pronghorn, protoceratops, protylopus, prurient, pteranodon, pteraspid, pterichthys, pterodactyl, pterosaur, puma, rabbit, raccoon, randy, rat, red deer, red squirrel,
reindeer, reptile, rhinoceros, roe, roe deer, roebuck, rough-and-ready,
rude, ruthless, sable, sadistic, salacious, sanguinary, sanguineous, satanic, satyric, sauropod, savage, scelidosaur, sensual, serpent, serval, sexual, sexy, shark, sharkish, sheep, shrew, shrew mole, sika, silver fox, skunk, slavering, sloth, smilodon, snake, snowshoe rabbit, springbok, squirrel, stoat, subhuman, suslik, swamp rabbit, swine, swinish, takin, tamandua, tamarin, tapir, tarpan, tatou, tatou peba, tatouay, tiger, tiger cat, timber wolf,
titanosaur, trachodon, tree shrew,
triceratops,
trilobite, troglodyte, troglodytic, truculent, tyrannosaur, uintathere, unchristian, uncivil, uncivilized, uncombed, uncouth, uncultivated, uncultured, unhuman, unkempt, unlicked, unpolished, unrefined, unspiritual, untamed, urus, vandal, varmint, vermin, vicious, viper, vole, wallaby, warthog, water buffalo, waterbuck, weasel, wharf rat, whelp, whistler, white fox, wild, wild ass, wild boar, wild
goat, wild man, wild ox, wildcat, wildebeest, wolf, wolfish, wolverine, wombat, wood rat, woodchuck, woolly mammoth,
worm, wrecker, yak, zebra, zebu, zoic, zooid, zooidal, zoologic, zoological, zoril