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Animal
Animals are a major group of organisms, classified as the kingdom Animalia
or Metazoa. In general they are multicellular, responsive to their
environment, and feed by consuming other organisms or parts of them. Their
body plan becomes fixed as they develop, usually early on in their
development as embryos, although some undergo a process of metamorphosis
later on.
The word "animal" comes from the Latin word animal, of which animalia is the
plural, 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. 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 575 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.
Groups of animals
The sponges (Porifera) 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.
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 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 four major
lineages:
1. Deuterostomes
2. Ecdysozoa
3. Platyzoa
4. Lophotrochozoa
In addition to these, 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, which
are invisible to the unaided eye, 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 lack a coelom, as do their closest relatives, the
microscopic Gastrotricha.
The other platyzoan phyla are 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 around the world concert their efforts on 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 organism, and 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.[1]
History of classification
Aristotle divided the living world between animals and plants, and this was
followed by Carolus Linnaeus 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' 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.
References
1. ^ N.H. Putnam, et al. (Jul 2007). "Sea anemone genome reveals ancestral
eumetazoan gene repertoire and genomic organization". Science 317 (5834):
86-94. DOI:10.1126/science.1139158.
* 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.
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