Shopping on line can be easy, simple and save you lots of money. It can also take a lot of your time, frustrate you, and result in unwanted purchases. Now the same can be said for regular high street shopping, but with the vast opportunity presented by the Internet it will pay you to spend a few minutes reading this and understanding how to better optimize your Animals shopping experience:

1. Compare - without doubt the biggest advantage that the Animals offers shoppers today is the ability to compare thousands of Animals at a time. This is a great thing, but not necessarily all the time! Too much can be daunting at times so take advantage of the great comparison sites and where possible let them do the hard work for you.

2. Research - if it has been said it will be on the internet. Ignorance is no longer a justifiable reason for buying the wrong thing. Take the time to research in detail everything that you could possible want to know about

3. Testimonials - don't know anybody that has bought a Animals? Wrong! If the Animals is good the internet will let you know. Use the Internet as a friend and get testimonials before you buy.

4. Questions - Got a question about Animals then search the Forums, FAQ's, Blogs etc. Don't be afraid to ask .....

5. Reputation - Never heard of the company selling Animals? Don't worry, no reason why you should know every company in the world, but you know someone that does! Use the internet to find out what people are saying about Animals and build up a picture of their reputation for sales, returns, customer service, delivery etc.

6. Returns - still worried that even after all of the above your Animals wont be what you want? Check out the returns policy. There is so much competition now that someone, somewhere is bound to offer the terms that you are comfortable with.

7. Feedback - happy with your Animals then let people know, after all you are depending on others people input in your buying decision, so why not give a little back.

8. Security - check for the yellow padlock on the Animals site before you buy, and the s after http:/ /i.e. https:// = a secure site

9. Contact - got a question about Animals, or want to leave a comment then check out the sites contact page. Reputable companies have them and respond.

10. Payment - ready to pay for your Animals, then use your credit card or PayPal! Be aware of companies that don't accept them, there may be genuine reasons but given the huge amount of choice you have when buying online there is no reason at all not to buy via credit card or PayPal.

{{Taxobox| color = pink| name = Animals| fossil_range = Ediacaran - Recent], Tawny Owl, Siberian Tiger, European garden spider, White-lipped snail, Green Sea Turtle, Solitary bee, Asian arowana, Barbary Macaque, Sawfish and a Marbled White (butterfly).]| unranked_regnum = Opisthokonta], Systema Naturae| subdivision_ranks = Phylum| subdivision =Subregnum Parazoa (alternatively)
Calcarea
Silicarea

Subregnum Eumetazoa -->, Iciligorgia schrammi, and a sea rod, Plexaurella nutans.

Animals are a major group of multicellular organisms, of the Kingdom (biology) Animalia or Metazoa. Their body plan becomes fixed as they develop, usually early on in their ontogeny as embryos, although some undergo a process of metamorphosis (biology) later on in their life. Most animals are motile - can move spontaneously and independently.

Etymology 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 eukaryote 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 fungus by lacking cell walls. All animals are motile, if only at certain life stages. Embryos pass through a Blastula, which is a characteristic exclusive to animals.

Structure With a few exceptions, most notably the sponges (Phylum Porifera), animals have bodies differentiated into separate biological tissue. These include muscles, which are able to contract and control locomotion, and nervous system, which sends and processes signals. There is also typically an internal digestion 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 animal 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 lung cell (biology) staining (biology) with fluorescent dyes undergoing mitosis, specifically early anaphase.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 spermatozoon or larger non-motile ovum. 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 (biology).

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 invagination 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, chemosynthesis archaea and eubacteria form the base of the food chain.

Origin and fossil record Animals are generally considered to have Evolution 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, fungus 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 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 organ (anatomy). 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:
  • Deuterostomes
  • Ecdysozoa
  • Platyzoa
  • 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 , Malurus cyaneusDeuterostomes 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 Chordate. 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 , Sympetrum flaveolumThe 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 genetic linkage 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.

    An analyse of the homoscleromorph sponge Oscarella carmela also suggests that the last common ancestor of sponges and the eumetazoan animals were more comlex than previously assumed. Mitochondrial Genome of the Homoscleromorph Oscarella carmela (Porifera, Demospongiae) Reveals Unexpected Complexity in the Common Ancestor of Sponges and Other Animals Oxford Journals

    Other model organisms belonging to the animal family include mouse (Mus musculus) and zebrafish (Danio rerio}.

    History of classification , known as the "father of modern taxonomy".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 Carolus Linnaeus original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Fish, Amphibia, bird, and Mammalia. Since then the last four have all been subsumed into a single phylum, the chordate, 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.

    See also

    Notes References

    External links

    {{Taxobox| color = pink| name = Animals| fossil_range = Ediacaran - Recent], Tawny Owl, Siberian Tiger, European garden spider, White-lipped snail, Green Sea Turtle, Solitary bee, Asian arowana, Barbary Macaque, Sawfish and a Marbled White (butterfly).]| unranked_regnum = Opisthokonta], Systema Naturae| subdivision_ranks = Phylum| subdivision =Subregnum Parazoa (alternatively)
    Calcarea
    Silicarea

    Subregnum Eumetazoa -->, Iciligorgia schrammi, and a sea rod, Plexaurella nutans.

    Animals are a major group of multicellular organisms, of the Kingdom (biology) Animalia or Metazoa. Their body plan becomes fixed as they develop, usually early on in their ontogeny as embryos, although some undergo a process of metamorphosis (biology) later on in their life. Most animals are motile - can move spontaneously and independently.

    Etymology 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 eukaryote 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 fungus by lacking cell walls. All animals are motile, if only at certain life stages. Embryos pass through a Blastula, which is a characteristic exclusive to animals.

    Structure With a few exceptions, most notably the sponges (Phylum Porifera), animals have bodies differentiated into separate biological tissue. These include muscles, which are able to contract and control locomotion, and nervous system, which sends and processes signals. There is also typically an internal digestion 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 animal 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 lung cell (biology) staining (biology) with fluorescent dyes undergoing mitosis, specifically early anaphase.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 spermatozoon or larger non-motile ovum. 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 (biology).

    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 invagination 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, chemosynthesis archaea and eubacteria form the base of the food chain.

    Origin and fossil record Animals are generally considered to have Evolution 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, fungus 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 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 organ (anatomy). 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:
  • Deuterostomes
  • Ecdysozoa
  • Platyzoa
  • 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 , Malurus cyaneusDeuterostomes 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 Chordate. 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 , Sympetrum flaveolumThe 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 genetic linkage 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.

    An analyse of the homoscleromorph sponge Oscarella carmela also suggests that the last common ancestor of sponges and the eumetazoan animals were more comlex than previously assumed. Mitochondrial Genome of the Homoscleromorph Oscarella carmela (Porifera, Demospongiae) Reveals Unexpected Complexity in the Common Ancestor of Sponges and Other Animals Oxford Journals

    Other model organisms belonging to the animal family include mouse (Mus musculus) and zebrafish (Danio rerio}.

    History of classification , known as the "father of modern taxonomy".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 Carolus Linnaeus original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Fish, Amphibia, bird, and Mammalia. Since then the last four have all been subsumed into a single phylum, the chordate, 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.

    See also

    Notes References

    External links



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