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Animal

2007 Schools Wikipedia Selection. Related subjects: General Biology

         iAnimal

                 Fossil range: Late Neoproterozoic - Recent

   Mammals, Birds, Reptiles and others.
   Mammals, Birds, Reptiles and others.
                         Scientific classification

   Domain:  Eukaryota
   Kingdom: Animalia
            Linnaeus, 1758

                                    Phyla

     * Placozoa (trichoplax)
     * Orthonectida (orthonectids)
     * Rhombozoa (dicyemids)

   Subregnum Parazoa
     * Porifera (sponges)

   Subregnum Eumetazoa
     * Radiata (unranked) ( radial symmetry)
          + Ctenophora (comb jellies)
          + Cnidaria (coral, jellyfish, anemones)
     * Bilateria (unranked) ( bilateral symmetry)
          + Acoelomorpha (basal)
          + Myxozoa (slime animals)
          + Chaetognatha (arrow worms)
          + Superphylum Deuterostomia (blastopore becomes anus)
               o Chordata (vertebrates, etc.)
               o Hemichordata (acorn worms)
               o Echinodermata (starfish, urchins)
          + Superphylum Ecdysozoa (shed exoskeleton)
               o Kinorhyncha (mud dragons)
               o Loricifera
               o Priapulida (priapulid worms)
               o Nematoda (roundworms)
               o Nematomorpha (horsehair worms)
               o Onychophora (velvet worms)
               o Tardigrada (water bears)
               o Arthropoda (insects, etc.)
          + Superphylum Platyzoa
               o Platyhelminthes (flatworms)
               o Gastrotricha (gastrotrichs)
               o Rotifera (rotifers)
               o Acanthocephala (acanthocephalans)
               o Gnathostomulida (jaw worms)
               o Micrognathozoa (limnognathia)
               o Cycliophora (pandora)
          + Superphylum Lophotrochozoa (trochophore larvae / lophophores)
               o Sipuncula (peanut worms)
               o Nemertea (ribbon worms)
               o Phoronida (horseshoe worms)
               o Bryozoa (moss animals)
               o Entoprocta (goblet worms)
               o Brachiopoda (brachipods)
               o Mollusca (molluscs)
               o Annelida (segmented worms)

   Animals are a major group of organisms, classified as the kingdom
   Animalia or Metazoa. In general they are multicellular, capable of
   locomotion, responsive to their environment, and feed by consuming
   other organisms. 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. More specifically, animals can be
   defined as heterotrophic eukaryotes without cell walls, which move
   through a blastula stage in early development.

   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". A common usage of the term may be limited to so-called lower
   animals (as in "brutes" or "beasts") and refer to humans only in a
   contemptuous or humourous context .

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 distinguishes them from plants and algae. They
   are also distinguished from plants, algae, and fungi because their
   cells lack cell walls.

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.

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.

Nutrition

   Animals are consumers that derive their energy from producers, who in
   turn derive their energy from a number of sources.

   Most animals grow by indirectly using the energy of sunlight. Plants
   use this energy to convert carbon dioxide (CO[2]) into simple sugars
   using a process known as photosynthesis. Starting with the (CO[2])
   molecules and water (H[2]O), photosynthesis converts the energy of
   sunlight into chemical energy stored in the bonds of glucose
   (C[6]H[12]O[6]) and releases oxygen (O[2]). These sugars are then used
   as the building blocks which allow the plant to grow. When animals eat
   these plants (or eat other creatures 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.

   Many animals who live close to hydrothermal vents and cold seeps on the
   ocean floor are not dependent on the energy of sunlight for their
   nourishment. Instead, chemosynthetic archaea and eubacteria form the
   base of the food chain. These creatures use the energy from compounds
   seeping from the vents to power the manufacture of sugars and other
   molecules, and animals live by either eating those microbes or
   harboring them within their tissues.

Origin and fossil record

   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
   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

   Orange elephant ear sponge, Agelas clathrodes, in foreground. Two
   corals in the background: a sea fan, Iciligorgia schrammi, and a sea
   rod, Plexaurella nutans.
   Enlarge
   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 magellanicus
   Enlarge
   Magellanic Penguin, Spheniscus magellanicus

   The 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. The extinct archaeocyathids, which have fused skeletons,
   may represent sponges or a separate phylum.

   Among the remaining phyla, known collectively as eumetazoans, 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.

   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, although this radial symmetry is superimposed upon the
   bilaterial symmetry of the embryo. Another exception is that 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 to the archenteron (the
   blastopore) 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. Echinoderms radially symmetric and
   exclusively marine, such as sea stars, sea urchins, and sea cucumbers.
   The most prevalent chordates are vertebrates, animals with backbones,
   which include fish, amphibians, reptiles, birds, and mammals.

   In addition to these, the deuterostomes also include the Hemichordata
   or acorn worms. The important fossil graptolites, although they are
   extinct today, may belong to this group.

   The Chaetognatha or arrow worms share some embryonic features with
   deuterostomes, but may actually be closer to the protostomes.

   The remaining three bilaterian groups are protostomes.

Ecdysozoa

   Yellow-winged Darter, Sympetrum flaveolum
   Enlarge
   Yellow-winged Darter, Sympetrum flaveolum

   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

   Bedford's Flatworm, Pseudobiceros bedfordi
   Enlarge
   Bedford's Flatworm, Pseudobiceros bedfordi

   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

   Big blue octopus, Octopus cyanea
   Enlarge
   Big blue octopus, Octopus cyanea

   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.

History of classification

   White's Tree Frog, Litoria caerulea.
   Enlarge
   White's Tree Frog, Litoria caerulea.

   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.

   Retrieved from " http://en.wikipedia.org/wiki/Animal"
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