Resumo de The Dying Animal

Animals have several characteristics that set them apart

from other living things. Animals are eukaryotic and

multicellular, 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.

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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 a nervous system, 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.

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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.

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 are generally considered to have evolved from
flagellate protozoa. Their closest living relatives are the
choanoflagellates, collared flagellates that have the same
structure as certain sponge cells do. Molecular studies
place them in a supergroup called the opisthokonts, which
also include the 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 575 million
years ago, and are known as the 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.

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Groups of animals

Orange elephant ear sponge, Agelas clathrodes, in
foreground. Two corals in the background: a sea fan,
Iciligorgia schrammi, and a sea rod, Plexaurella nutans.
Magellanic Penguin, Spheniscus magellanicusThe sponges
(Porifera) diverged from other animals early. As mentioned,
they lack the complex organization found in most other
phyla. Their cells are differentiated, but 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.

Among the eumetazoan phyla, two are radially symmetric and
have digestive chambers with a single opening, which serves
as both the mouth and the anus. These are the Cnidaria,
which include sea anemones, corals, and jellyfish, and the
Ctenophora or comb jellies. 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 phylum Placozoa is similar,
but individuals do not have a permanent digestive chamber.

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