Search Britannica
Click here to search
SUBSCRIBE
Login
Ask the ChatbotGames & QuizzesHistory & SocietyScience & TechBiographiesAnimals &
NatureGeography & TravelArts & CultureProConMoneyVideos
Science
Biology
fallow deer (Dama dama) The fallow deer (Dama dama) has been
introduced in many areas ever since the Neolithic Period.
animal
organism
Ask the Chatbot a Question
More Actions
Also known as: Animalia
Written by
Virginia C. Maiorana,
Leigh M. Van Valen•All
Fact-checked by
The Editors of Encyclopaedia Britannica
Last Updated: Mar 21, 2025 • Article History
Table of Contents
Key People: Charles Elton Carl E. Akeley Karl P. Schmidt Spencer Fullerton
Baird Ross Granville Harrison
(Show more)
Related Topics: animal social behaviour reptile sh Eumetazoa coelomate
See all related content
Top Questions
What is an animal?
What are the two major groups of animals?
When did animals rst appear?
fi
fi
1 of 2
Survival struggles in a meerkat colonyA dominant female meerkat
expelling a subordinate from the pack.See all videos for this article
2 of 2
freshwater jelly shFreshwater jellyfish (Craspedacusta sowerbyi).
animal, (kingdom Animalia), any of a group of
multicellular eukaryotic organisms (i.e., as distinct
from bacteria, their deoxyribonucleic acid, or DNA, is
contained in a membrane-bound nucleus). They are thought
to have evolved independently from the unicellular
eukaryotes. Animals differ from members of the two other
kingdoms of multicellular eukaryotes, the plants (Plantae)
and the fungi (Mycota), in fundamental variations
in morphology and physiology. This is largely because
animals have developed muscles and hence mobility, a
characteristic that has stimulated the further development
of tissues and organ systems.
Animals dominate human conceptions of life on Earth not
simply by their size, abundance, and sheer diversity but also
by their mobility, a trait that humans share. So integral is
movement to the conception of animals that sponges, which
lack muscle tissues, were long considered to be plants. Only
after their small movements were noticed in 1765 did the
animal nature of sponges slowly come to be recognized.
fi
gray whale (Eschrichtius robustus)Gray whale (Eschrichtius robustus)
breaching.
In size animals are outdone on land by plants, among whose
foliage they may often hide. In contrast, the photosynthetic
algae, which feed the open oceans, are usually too small to
be seen, but marine animals range to the size of whales.
Diversity of form, in contrast to size, only impinges
peripherally on human awareness of life and thus is less
noticed. Nevertheless, animals represent three-quarters or
more of the species on Earth, a diversity that reflects the
flexibility in feeding, defense, and reproduction
which mobility gives them. Animals follow virtually every
known mode of living that has been described for the
creatures of Earth.
Animals move in pursuit of food, mates, or refuge from
predators, and this movement attracts attention and
interest, particularly as it becomes apparent that the
behaviour of some creatures is not so very different
from human behaviour. Other than out of simple curiosity,
humans study animals to learn about themselves, who are a
very recent product of the evolution of animals.
The animal kingdom
The major parts of an animal cell explainedFrom teeny-tiny ants to 200-
ton whales, all members of the animal kingdom are composed of cells.
(more)
See all videos for this article
Animals evolved from unicellular eukaryotes. The presence
of a nuclear membrane in eukaryotes permits separation of
the two phases of protein synthesis: transcription (copying)
of deoxyribonucleic acid (DNA) in the nucleus and
translation (decoding) of the message into protein in the
cytoplasm. Compared to the structure of the bacterial cell,
this gives greater control over which proteins are produced.
Such control permits specialization of cells, each with
identical DNA but with the ability to control finely which
genes successfully send copies into the cytoplasm. Tissues
and organs can thus evolve. The semirigid cell walls found in
plants and fungi, which constrain the shape and hence the
diversity of possible cell types, are absent in animals. If they
were present, nerve and muscle cells, the focal point of
animal mobility, would not be possible.
Britannica Quiz
Deadliest Animals Quiz
A de nition of animals
fi
animal cell and plant cellCytoplasm is contained within cells in the space
between the cell membrane and the nuclear membrane.
(more)
A characteristic of members of the animal kingdom is the
presence of muscles and the mobility they afford. Mobility is
an important influence on how an organism obtains
nutrients for growth and reproduction. Animals typically
move, in one way or another, to feed on other living
organisms, but some consume dead organic matter or even
photosynthesize by housing symbiotic algae. The type of
nutrition is not as decisive as the type of mobility in
distinguishing animals from the other two multicellular
kingdoms. Some plants and fungi prey on animals by using
movements based on changing turgor pressure in key cells,
as compared with the myofilament-based mobility seen in
animals. Mobility requires the development of vastly
more elaborate senses and internal communication than are
found in plants or fungi. It also requires a different mode of
growth: animals increase in size mostly by expanding all
parts of the body, whereas plants and fungi mostly extend
their terminal edges.
All phyla of the animal kingdom, including sponges,
possess collagen, a triple helix of protein that binds cells into
tissues. The walled cells of plants and fungi are held together
by other molecules, such as pectin. Because collagen is not
found among unicellular eukaryotes, even those forming
colonies, it is one of the indications that animals arose once
from a common unicellular ancestor.
The muscles that distinguish animals from plants or fungi
are specializations of the actin and myosin microfilaments
common to all eukaryotic cells. Ancestral sponges, in fact,
are in some ways not much more complex than aggregations
of protozoans that feed in much the same way. Although the
sensory and nervous system of animals is also made of
modified cells of a type lacking in plants and fungi, the basic
mechanism of communication is but a specialization of a
chemical system that is found in protists, plants, and fungi.
The lines that divide an evolutionary continuum are rarely
sharp.
Mobility constrains an animal to maintain more or less the
same shape throughout its active life. With growth,
each organ system tends to increase roughly
proportionately. In contrast, plants and fungi grow by
extension of their outer surfaces, and thus their shape is
ever changing. This basic difference in growth patterns has
some interesting consequences. For example, animals can
rarely sacrifice parts of their bodies to satisfy the appetites
of predators (tails and limbs are occasionally exceptions),
whereas plants and fungi do so almost universally.
History of classi cation
Except perhaps for the possession of collagen,
the criteria used above to distinguish animals from other
fi
forms of life are not absolute. The first catalogs of
animal diversity were based on overall form and
similarity. Aristotle and other early biologists regarded all
organisms as part of a great chain, divisions of which were
more or less arbitrary. The 18th-century Swedish
botanist Carolus Linnaeus divided all animals into six
classes: Mammalia, Aves, Amphibia (including
reptiles), Pisces, Insecta (Arthropoda), and Vermes (other
invertebrates). In the early 1800s the French
zoologist Georges Cuvier recognized that vertebrates were
substantially different from invertebrates, and he divided
most animals on the basis of form and function into four
branches: vertebrates, arthropods (articulates), mollusks,
and radiates (animals with radial symmetry). Cuvier’s
divisions formed the basis for all subsequent classifications.
Just after Cuvier’s classification, the French
naturalist Étienne Geoffroy Saint-Hilaire outlined the
importance of homologous structures. Homology is
correspondence between features caused by continuity of
information. Thus, a bird’s wing is homologous to
a bat’s wing insofar as both are forelimbs, but they are not
homologous as wings. Homologous structures need not
resemble each other; for example, the three bones in the
middle ear of humans are homologous to three bones in the
jaw apparatus in fishes because the genetic and
developmental information controlling them has been
continuous through evolutionary change.
Before evolution was generally accepted, homologies among
different animals, when they were recognized at all, were
regarded as aspects of God’s pattern. Evolution provided a
testable explanation for homologies. By carefully tracing
selected homologies, it has been possible to show that
previously proposed classifications established
inappropriate relationships based solely on form or
function, or both; for example, the radial symmetry of
starfishes is not homologous to that of coelenterates (such as
jellyfish).
Protozoans were once considered to be animals because they
move and do not photosynthesize. Closer study has shown,
though, that their movement is by means of nonmuscular
structures (cilia, flagella, or pseudopods) and that
photosynthesis in them has often been lost and gained.
Protozoans do not, therefore, form a natural group but with
algae form a eukaryotic kingdom separate from plants and
animals, called Protista.
Like plants and animals, fungi arose from protists and are
now accorded a kingdom of their own.
