The word "animal" comes from the Latin word animalis, meaning "havingbreath". In everyday colloquial
usage, the word often refers to non-human members of kingdom Animalia. Sometimes, only closer
relatives of humans such as mammals and other vertebrates are meant in colloquial use. The biological
definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse
as sponges, jellyfish, insects and humans.
Animals have several characteristics that set them apart from other living things. Animals
are eukaryotic and multicellular, which separates them frombacteria 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 rigid cell walls. All animals
are motile, if only at certain life stages. In most animals, embryospass through a blastula stage, which is a
characteristic exclusive to animals.
With a few exceptions, most notably the sponges (Phylum Porifera) andPlacozoa, animals
have bodies differentiated into separate tissues. These include muscles, which are able to contract and
control locomotion, and nerve tissues, which send and process signals. Typically, there is also an
internaldigestive 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,
Nearly all animals undergo some form of sexual reproduction. They have a few specialized reproductive
cells, which undergo meiosis to produce smaller, motilespermatozoa or larger, non-motileova. These fuse
to form zygotes, which develop into new individuals.
Many animals are also capable ofasexual reproduction. This may take place through parthenogenesis,
where fertile eggs are produced without mating, budding, orfragmentation.
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