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• Etymology
• Characteristics
• Structure
• Reproduction and Development
• Origin and Fossil Record
• Groups of Animals
• Deuterostomes
• Ecdysozoa
• Platyzoa
• Lophotrochozoa
• Model Organisms
• History of Classification
• Photos
• Map
• Taxonomy
• Phyla
• References
• Footnotes
• Sources
• Keep Exploring

Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. 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 in their life. Most animals are motile - they can move spontaneously and independently. Animals are heterotrophs - they are dependent on other organisms (e.g. plants) for sustenance.

Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago.

Etymology

The word "animal" comes from the Latin word animale, neuter of animalis, 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.^[1]

Characteristics

Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and usually multicellular^[2] (although see Myxozoa), which separates them from bacteria and most protists. They are heterotrophic,^[3] 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.^[4] All animals are motile,^[5] ifonly 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 usinga 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 610 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 appearanceduring 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. However some paleontologists and geologists would suggest that animals appeared much earlier than previously thought, possibly even as early as 1 billion years ago. Trace fossils such as tracks and burrows found in Tonian era strata in India indicate the presence of triploblastic worm like metazoans roughly as large (about 5mm wide) and complex as earthworms.^[6] In addition during the beginning of the Tonian period around 1 billion years ago (roughly the same time that the trace fossils previously discussed in this article date back to) there was a decrease in Stromatolite diversity which may indicate the appearance of grazing animals during this time as Stromatolites also increased in diversity shortly after the end-Ordovician and end-Permian rendered large amounts of grazing marine animals extinct and decreased shortly after their populations recovered. However some other scientists doubt that these fossils are authentic and have suggested these trace fossils are just the result of natural processes such as erosion.^{[citation needed]}

Groups of Animals

The sponges (Porifera) were long thought to have 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. However, a phylogenomic study in 2008 of 150 genes in 21 genera^[7] revealed that it is the Ctenophora or comb jellies which are the basal lineage of animals, at least among those 21 phyla. The authors speculate that sponges—or at least those lines of sponges they investigated—are not so primitive, but may instead be secondarily simplified.

Among the other phyla, the Ctenophora and the Cnidaria, which includes sea anemones, corals, and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus. 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 two major lineages: the Deuterostomes and Protostomes, which includes the Ecdysozoa, Platyzoa, and Lophotrochozoa. In addition, 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, 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.^[8]

A number of parasites are included in this group, such as the flukes and tapeworms. Flatworms are acoelomates, or lack a body cavity, as do their closest relatives, the microscopic Gastrotricha.^[9]

The other platyzoan phyla are mostly 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.^[10] 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.^[11][12] 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,^[13] because they are both segmented. Now this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.^[14]

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.^[15] These were traditionally grouped together as the lophophorates.^[16] but it now appears they are paraphyletic,^[17] some closer to the Nemertea and some to the Mollusca and Annelida.^[18][19] 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.^[20]

Model Organisms

Because of the great diversity found in animals, it is more economical for scientists to study 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.^[21]

An analysis of the homoscleromorph sponge Oscarella carmela also suggests that the last common ancestor of sponges and the eumetazoan animals was more complex than previously assumed.^[22]

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

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's 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.

Photos

Map

Taxonomy

Phyla

Acanthocephala

The Acanthocephala (Greek akanthos, thorn + kephale, head) is a phylum of parasitic worms known as acanthocephales, thorny-headed worms, or spiny-headed worms, characterised by the presence of an evertable proboscis, armed with spines, which it uses to pierce and hold the gut wall of its host. Acanthocephalans typically have complex life cycles, involving a number of hosts, including invertebrates, fishes, amphibians, birds, and mammals. About 1150 species have been described. [more]

At least 1,052 species and subspecies belong to the Phylum Acanthocephala.

More info about the Phylum Acanthocephala may be found here.

Acoelomorpha

At least 8 species and subspecies belong to the Phylum Acoelomorpha.

More info about the Phylum Acoelomorpha may be found here.

Annelida

The annelids, collectively called Annelida (from Latin anellus "little ring"), are a large phylum of animals comprising the segmented worms, with about 15,000 modern species including the well-known earthworms and leeches. They are found in most wet environments, and include many terrestrial, freshwater, and especially marine species (such as the polychaetes), as well as some which are parasitic or mutualistic. They range in length from under a millimeter to over 3 meters (the seep tube worm Lamellibrachia luymesi). [more]

At least 10,925 species and subspecies belong to the Phylum Annelida.

More info about the Phylum Annelida may be found here.

Arthropoda

Arthropods are animals belonging to the Phylum Arthropoda (from Greek ἄ�θ�ον arthron, "joint", and ποδός podos, "foot") and include the insects, arachnids, crustaceans and allies. Arthropods are characterized by the possession of a segmented body with appendages on at least one segment. They have a dorsal heart and a ventral nervous system. All arthropods are covered by a hard exoskeleton made of chitin, a polysaccharide, which provides physical protection and resistance to desiccation. Arthropods grow by shedding this covering in what are termed molts. [more]

At least 991,394 species and subspecies belong to the Phylum Arthropoda.

More info about the Phylum Arthropoda may be found here.

Brachiopoda

Brachiopods (from Latin bracchium, arm + New Latin -poda, foot) are a small phylum of benthic invertebrates. Also known as lamp shells (or lampshells), "brachs" or Brachiopoda, they are sessile, two-valved, marine animals with an external morphology superficially resembling pelecypods (for instance, clams) of phylum Mollusca to which they are not closely related. It is estimated by paleobiologists that 99 percent of all documented lamp-shell species are both fossils and extinct. [more]

At least 11,517 species and subspecies belong to the Phylum Brachiopoda.

More info about the Phylum Brachiopoda may be found here.

Bryozoa

Bryozoans are tiny colonial animals that generally build stony skeletons of calcium carbonate, superficially similar to coral, although some species lack any calcification in the form of the colony and are comprised of mucilaginous structure and lack . They are also known as moss animals (which is the literal Greek translation) or sea mats. They generally prefer warm, tropical waters but are known to occur worldwide. There are about 8,000 living species, with several times that number of fossil forms known. [more]

At least 2,873 species and subspecies belong to the Phylum Bryozoa.

More info about the Phylum Bryozoa may be found here.

Cephalorhyncha

Scalidophora is a group of marine pseudocoelomate invertebrates, consisting of the three phyla Kinorhyncha, Priapulida, and Loricifera. The members of the group share a number of characteristics, including introvert larvae and moulting of the cuticle (ecdysis). Their closest relatives are thought to be the Panarthropoda and Nematoda; they are thus placed in the group Ecdysozoa. [more]

At least 76 species and subspecies belong to the Phylum Cephalorhyncha.

More info about the Phylum Cephalorhyncha may be found here.

Chaetognatha

Chaetognatha is a phylum of predatory marine worms that are a major component of plankton worldwide. About 20% of the known species are benthic and can attach to algae or rocks. They are found in all marine waters from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped. Some deep-sea species are orange. They range in size from 2 mm to 12 cm. The common term for the phylum is Arrow Worms. There are more than 120 modern species assigned to over 20 genera. Despite the limited diversity of species, the number of individuals is staggering. [more]

At least 292 species and subspecies belong to the Phylum Chaetognatha.

More info about the Phylum Chaetognatha may be found here.

Chordata

Chordates (phylum Chordata) are a group of animals that includes the vertebrates, together with several closely related invertebrates. They are united by having, at some time in their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail. Some scientists argue that the true qualifier should be pharyngeal pouches rather than slits.^{[citation needed]} [more]

At least 179,023 species and subspecies belong to the Phylum Chordata.

More info about the Phylum Chordata may be found here.

Cnidaria

Cnidaria (pronounced /naɪˈdɛəriə/) is a phylum containing some 11,000 species of apparently simple animals found exclusively in aquatic, mostly marine, environments. On the other hand, their biochemistry and genetic makeup reveal that even the stationary starlet sea anemone is much more complex than it appears at the first glance. [more]

At least 20,020 species and subspecies belong to the Phylum Cnidaria.

More info about the Phylum Cnidaria may be found here.

Coelenterata

Coelenterata is an obsolete yet common term encompassing two animal phyla, the Ctenophora (comb jellies) and the Cnidaria (coral animals, true jellies, sea anemones, sea pens, and their allies). The taxon name comes from the Greek "koilos" ("hollow"), referring to the hollow body cavity common to these two phyla. They have very simple tissue organization, with only two layers of cells, external and internal. [more]

At least 696 species and subspecies belong to the Phylum Coelenterata.

More info about the Phylum Coelenterata may be found here.

Ctenophora

The phylum Ctenophora (pronounced /tɨˈnɒfərə/), commonly known as Comb Jellies, is a phylum that includes the sea gooseberry (Pleurobrachia pileus) and Venus' girdle (Cestum veneris). Classically grouped with Cnidaria (jellyfish) in the Coelenterata infrakingdom, ctenophores have recently been identified as the most basal lineage of animals known. [more]

At least 203 species and subspecies belong to the Phylum Ctenophora.

More info about the Phylum Ctenophora may be found here.

Cycliophora

Symbion is a genus of peculiar microscopic animals, with no obvious close relatives, and which was therefore given its own phylum, called Cycliophora. [more]

More info about the Phylum Cycliophora may be found here.

Dicyemida

Rhombozoa, or Dicyemida, is a phylum of tiny parasites that live in the renal appendages of cephalopods. Although the name Dicyemida precedes Rhombozoa in usage, and is preferred by most contemporary authors, Rhombozoa still enjoys much popular support. [more]

At least 80 species and subspecies belong to the Phylum Dicyemida.

More info about the Phylum Dicyemida may be found here.

Echinodermata

Echinoderms (Phylum Echinodermata, from the Greek for spiny skin) are a phylum of marine animals found at all ocean depths. Aside from the problematic Arkarua, the first definitive members of the phylum appeared near the start of the Cambrian period, and contains about 7,000 living species, making it the second largest grouping of deuterostomes, after the chordates; they are the largest phylum without freshwater or terrestrial representatives. [more]

At least 9,039 species and subspecies belong to the Phylum Echinodermata.

More info about the Phylum Echinodermata may be found here.

Echiura

The Echiura, or spoon worms, are a small group of marine animals. They are often considered to be a group of annelids, although they lack the segmented structure found in other members of that group, and so may also be treated as a separate phylum. However, phylogenetic analyses of DNA sequences place echiurans and pogonophorans within the Annelida. The Echiura fossilise poorly and the earliest known specimen is from the Upper Carboniferous (called the Pennsylvanian in North America). However, U-shaped fossil burrows that could be Echiuran have been found dating back to the Cambrian. [more]

At least 211 species and subspecies belong to the Phylum Echiura.

More info about the Phylum Echiura may be found here.

Ectoprocta

Bryozoans are tiny colonial animals that generally build stony skeletons of calcium carbonate, superficially similar to coral, although some species lack any calcification in the form of the colony and are comprised of mucilaginous structure and lack . They are also known as moss animals (which is the literal Greek translation) or sea mats. They generally prefer warm, tropical waters but are known to occur worldwide. There are about 8,000 living species, with several times that number of fossil forms known. [more]

At least 1,427 species and subspecies belong to the Phylum Ectoprocta.

More info about the Phylum Ectoprocta may be found here.

Entoprocta

Entoprocta (Gr. entos inside + proktos anus) is a phylum of small aquatic animals, ranging in size from 0.5 mm to 5.0 mm. They have a lophophore, and as their name suggests, are distinguished from other lophophorates by the position of the anus inside the ring of cilia rather than outside. Other names include goblet worm and kamptozoan. [more]

At least 170 species and subspecies belong to the Phylum Entoprocta.

More info about the Phylum Entoprocta may be found here.

Euglenophycota

More info about the Phylum Euglenophycota may be found here.

Gastrotricha

The gastrotrichs (from Greek gaster "stomach" and thrix "hair") are a phylum of microscopic (0.06-3.0 mm) animals abundant in fresh water and marine environments. Most fresh water species are part of the periphyton and benthos. Marine species are found mostly interstitially in between sediment particles. They are bilaterally symmetric, with a complete gut. They demonstrate eutely, with development proceeding to a particular number of cells, and further growth coming only from an increase in cell size. The body is covered with cilia, especially about the mouth and on the ventral surface, and has two terminal projections with cement glands that serve in adhesion. This is a double-gland system where one gland secretes the glue and another secretes a de-adhesive to sever the connection. Like many microscopic animals, their locomotion is primarily powered by hydrostatics, and they reproduce entirely by parthenogenesis. Originally they were thought to have a body cavity (pseudocoel), but this was an artifact created by preservation methods, and they are now known to be acoelomate. Their relationship to other phyla is unclear. Morphology suggests that they are close to the Gnathostomulida, the Rotifera, or the Nematoda. On the other hand genetic studies place them as close relatives of the Platyhelminthes, the Ecdysozoa or the Lophotrochozoa. About 700 species have been described. The average life span of a gastrotrich is very short - about three days. [more]

At least 771 species and subspecies belong to the Phylum Gastrotricha.

More info about the Phylum Gastrotricha may be found here.

Gnathostomulida

Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. Most measure between 0.5 and 1 mm long. Like flatworms they have a ciliated epidermis, but are unique in having but one cilium per cell. They have no body cavity, and no circulatory or respiratory system. Each gnathostomulid is simultaneously hermaphrodite, possessing an ovary and a testis. They are characterized by a specialized, muscular jaw, which they use to scrape smaller organisms off of the grains of sand that make up their anoxic seabed mud habitat. This bilaterally symmetrical pharynx with its complex cuticular mouth parts make them appear closely related to rotifers and their allies, together making up the Gnathifera. [more]

At least 100 species and subspecies belong to the Phylum Gnathostomulida.

More info about the Phylum Gnathostomulida may be found here.

Haptophyta

The haptophytes, classed either as the Prymnesiophyta or Haptophyta, are a phylum of algae. The chloroplasts are pigmented similarly to those of the heterokonts, such as golden algae, but the structure of the rest of the cell is different, so it may be that they are a separate line whose chloroplasts are derived from similar endosymbionts. [more]

At least 838 species and subspecies belong to the Phylum Haptophyta.

More info about the Phylum Haptophyta may be found here.

Hemichordata

Hemichordata is a phylum of worm-shaped marine deuterostome animals, generally considered the sister group of the echinoderms. They date back to the Lower or Middle Cambrian and include an important class of fossils called graptolites, most of which became extinct in the Carboniferous. They seem to have a primitive form of notochord, formed from a diverticulum of the foregut called a stomochord, but this is most likely the result of convergent evolution. A hollow neural tube exists among some species (at least in early life), probably a primitive trait they share with the common ancestor of chordata and the rest of the deuterostomes. [more]

At least 876 species and subspecies belong to the Phylum Hemichordata.

More info about the Phylum Hemichordata may be found here.

Kinorhyncha

Kinorhyncha (Gr. kīne� 'move' + rhynchos 'snout') is a phylum of small (1 mm or less) marine pseudocoelomate invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos. They are also called mud dragons. [more]

At least 134 species and subspecies belong to the Phylum Kinorhyncha.

More info about the Phylum Kinorhyncha may be found here.

Loricifera

Loricifera (from Latin, lorica, corselet + Greek, phora, bearing) is a small phylum of marine sediment-dwelling animals with twenty-two described species, in eight genera. Aside from these described species, there are approximately 100 more which have been collected and not yet described. They are characterised by a protective outer case called a lorica and their habitat, which is in the spaces between marine gravel to which they attach themselves. The phylum was discovered in 1983 by Reinhardt Kristensen, in Roscoff, France. They are among the very latest of discovered groups of Metazoans. They attach themselves quite firmly to the substratum, and so remained undiscovered for so long. The first specimen was collected in the 1970s, and later described in 1983. They are found at all depths, in different sediment types, and in all latitudes. [more]

At least 5 species and subspecies belong to the Phylum Loricifera.

More info about the Phylum Loricifera may be found here.

Mandibulata

In arthropods, the mandible is either of a pair of arthropod mouthparts used for biting, cutting and holding food. Mandibles are often simply referred to as jaws. The arthropods with mandibles form the clade Mandibulata, comprising the extant subphyla Myriapoda, Crustacea and Hexapoda. Mandibulata is currently believed to be the sister group to the rest of arthropods, the clade Arachnomorpha (Chelicerata+Trilobita). The mandibulates constitute the largest and most varied arthropod group. [more]

More info about the Phylum Mandibulata may be found here.

Mollusca

The molluscs (British spelling) or mollusks (American spelling) are members of the very large and diverse phylum of invertebrate animals known as Mollusca. There are some 112,000 species within this phylum. The scientific study of molluscs is known as malacology. [more]

At least 166,537 species and subspecies belong to the Phylum Mollusca.

More info about the Phylum Mollusca may be found here.

Myxozoa

The Myxozoa (etymology: Greek: myx- "slime" or "mucus" + zoa "animals") are a group of parasitic animals of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. Infection occurs through valved spores. These contain one or two sporoblast cells and one or more polar capsules that contain filaments which anchor the spore to its host. The sporoblasts are then released as a motile form, called an amoebula, which penetrates the host tissues and develops into one or more multinucleate plasmodia. Certain nuclei later pair up, one engulfing another, to form new spores. [more]

At least 121 species and subspecies belong to the Phylum Myxozoa.

More info about the Phylum Myxozoa may be found here.

Nemata

The nematodes or roundworms (Phylum Nematoda from Greek νῆμα (nema): "thread" + -ώδη -ode "like") are one of the most common phyla of animals, with over 80,000 different described species (over 15,000 are parasitic). They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as Antarctica and oceanic trenches. Further, there are a great many parasitic forms, including pathogens in most plants, animals, and also in humans. [more]

At least 2,756 species and subspecies belong to the Phylum Nemata.

More info about the Phylum Nemata may be found here.

Nematoda

The nematodes or roundworms (Phylum Nematoda from Greek νῆμα (nema): "thread" + -ώδη -ode "like") are one of the most common phyla of animals, with over 80,000 different described species (over 15,000 are parasitic). They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as Antarctica and oceanic trenches. Further, there are a great many parasitic forms, including pathogens in most plants, animals, and also in humans. [more]

At least 6,449 species and subspecies belong to the Phylum Nematoda.

More info about the Phylum Nematoda may be found here.

Nematomorpha

Nematomorpha (sometimes called Gordiacea, and commonly known as Horsehair worms or Gordian worms) are a phylum of parasitic animals which are morphologically and ecologically similar to nematode worms, hence the name. They are, on average, 1 meter long, and 1 to 3 millimetres in diameter. Horsehair worms can be discovered in damp areas such as watering troughs, streams, puddles, and cisterns. The adult worms are free living, but the larvae are parasitic on beetles, cockroaches, grasshoppers and crustaceans. About 320 species have been described. [more]

At least 113 species and subspecies belong to the Phylum Nematomorpha.

More info about the Phylum Nematomorpha may be found here.

Nemertea

Nemertea is a phylum of invertebrate animals also known as ribbon worms or proboscis worms . Most of the 1,400 or so species are marine, with a few living in fresh water and a small number of terrestrial forms; they are found in all marine habits, and throughout the world's oceans . Nemerteans are named for Nemertes, one of the Nereids of Greek mythology, and alternative spellings for the phylum have included Nemertini, Nemertinea and Nemertea. [more]

At least 565 species and subspecies belong to the Phylum Nemertea.

More info about the Phylum Nemertea may be found here.

Onychophora

The velvet worms (Onychophora - literally "claw bearers") form a clade within the Ecdysozoa and can be simply described as "worms with legs". Most common in the Southern Hemisphere, they prey on smaller animals such as insects, which they catch by squirting a sticky slime. In modern zoology they are particularly renowned for their curious mating behaviour and the fact that they bear live young. The Lobopoda, possible ancestors of velvet worms from the Cambrian period, are of great interest in paleontology. [more]

At least 48 species and subspecies belong to the Phylum Onychophora.

More info about the Phylum Onychophora may be found here.

Orthonectida

Orthonectida is a small phylum of poorly-known parasites of marine invertebrates that are among the simplest of multi-cellular organisms. Members of this phylum are known as orthonectids. [more]

At least 27 species and subspecies belong to the Phylum Orthonectida.

More info about the Phylum Orthonectida may be found here.

Phoronida

Phoronids ('Phoronida'), commonly known as horseshoe worms, are a relatively small animal phylum: twenty species are known, in two genera, Phoronis and Phoronopsis. Phoronids are worm-shaped, but with a gut that loops and exits the body near the mouth, instead of running the length of the animal, as in annelids (and many vertebrates). They are found in all oceans and seas (except the polar seas) and all species have wide geographical ranges and most are cosmopolitan. They occur at depths ranging to about 400 metres, but mainly between 0 to 70 metres. The life span is thought to be about one year. The adults secrete chitinous tubes in which to live. These tubes can be buried in the mud or sand that makes up the sea bed or can be resting on the surface of a rocky substrate, in this case they tend to live in colonies and their tubes become twisted around each other for support to form a large impenetrable mass. Some species can dissolve away holes in rocks such as limestone, calcareous seashells or even cement piers, they then live in these holes which they line with their secreted tubes. [more]

At least 22 species and subspecies belong to the Phylum Phoronida.

More info about the Phylum Phoronida may be found here.

Placozoa

Trichoplax adhaerens is a simple balloon-like marine animal with a body cavity filled with pressurized fluid. It is given its own phylum, called Placozoa, sometimes referred to commonly as the tablet animals, which is a direct translation from the Greek scientific name; the only other species assigned to this taxon, Treptoplax reptans, was described in 1896 and has not been seen since, leading to doubts about its existence. [more]

More info about the Phylum Placozoa may be found here.

Platyhelminthes

The flatworms (Phylum Platyhelminthes from the Greek platy, meaning "flat" and helminth, meaning worm) are a phylum of relatively simple soft-bodied invertebrate animals. With about 25,000 known species they are the largest phylum of acoelomates. Flatworms are found in marine, freshwater, and even damp terrestrial environments. A troublesome terrestrial example is the New Zealand flatworm, Arthurdendyus triangulatus, which rapidly colonized large areas of Ireland and Scotland since its unintentional introduction in the 1960s and has since destroyed most of the indigenous earthworms . Most flatworms are free-living, but many are parasitic. There are four classes: Trematoda (flukes), Cestoda (tapeworms), Monogenea, and Turbellaria. [more]

At least 7,211 species and subspecies belong to the Phylum Platyhelminthes.

More info about the Phylum Platyhelminthes may be found here.

Porifera

The sponges or poriferans (from Latin porus "pore" and ferre "to bear") are animals of the phylum Porifera (pronounced /pɒˈrɪfərə/). Porifera translates to "Pore-bearer". They are primitive, sessile, mostly marine, water dwelling filter feeders that pump water through their bodies to filter out particles of food matter. Sponges represent the simplest of animals. With no true tissues (parazoa), they lack muscles, nerves, and internal organs. Their similarity to colonial choanoflagellates shows the probable evolutionary jump from unicellular to multicellular organisms. However, recent genomic studies suggest they are not the most ancient lineage of animals, but may instead be secondarily simplified. [more]

At least 13,017 species and subspecies belong to the Phylum Porifera.

More info about the Phylum Porifera may be found here.

Prasinophyta

At least 126 species and subspecies belong to the Phylum Prasinophyta.

More info about the Phylum Prasinophyta may be found here.

Priapulida

Priapulida (priapulid worms or penis worms, from Gr. pri�pos 'Priapus' + Lat. -ul-, diminutive) are a phylum of marine worms with an extensible spiny proboscis. Priapulid fossils are known at least as far back as the Middle Cambrian. Their nearest relatives are probably Kinorhyncha and Loricifera with which they constitute the taxon Scalidophora. Besides arthropods and velvet worms, it is only among Priapulida that we can find members of the Ecdysozoa which are relatively large in size. They were likely the major predators of the Cambrian period. There are 16 known species of Priapulid worms. [more]

At least 14 species and subspecies belong to the Phylum Priapulida.

More info about the Phylum Priapulida may be found here.

Protozoa

Protozoa (in Greek proto = first and zoa = animals) are unicellular eukaryotes, (singular protozoan). While there is no exact definition of the term, most scientists use protozoan to refer to a unicellular heterotrophic protist. [more]

At least 3,320 species and subspecies belong to the Phylum Protozoa.

More info about the Phylum Protozoa may be found here.

Rotifera

The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by John Harris in 1696 (Hudson and Gosse, 1886).^{[citation needed]} Leeuwenhoek is mistakenly given credit for being the first to describe rotifers but Harris had produced sketches in 1703.^{[citation needed]} Most rotifers are around 0.1-0.5 mm long, and are common in freshwater throughout the world with a few saltwater species. Rotifers may be free swimming and truly planktonic, others move by inchworming along the substrate whilst some are sessile, living inside tubes or gelatinous holdfasts. About 25 species are colonial (e.g. Sinantherina semibullata), either sessile or planktonic. [more]

At least 2,487 species and subspecies belong to the Phylum Rotifera.

More info about the Phylum Rotifera may be found here.

Sarcomastigophora

The phylum Sarcomastigophora includes many unicellular or colonial, autotrophic, or heterotrophic organisms. The two main sub-phyla are Mastigophora and Sarcodina. [more]

At least 1,435 species and subspecies belong to the Phylum Sarcomastigophora.

More info about the Phylum Sarcomastigophora may be found here.

Sipuncula

The Sipuncula or Sipunculida, sipunculid worms or peanut worms, are a phylum containing 144-320 specieskulones [more]

At least 504 species and subspecies belong to the Phylum Sipuncula.

More info about the Phylum Sipuncula may be found here.

Tardigrada

Tardigrades (commonly known as water bears) comprise the phylum Tardigrada. They are small, segmented animals, similar and probably related to the arthropods. Tardigrades were first described by Johann August Ephraim Goeze in 1773 (kleiner Wasserbär = little water bear). The name Tardigrada means "slow walker" and was given by Spallanzani in 1777. The biggest adults may reach a body length of 1.5 mm, the smallest below 0.1 mm. Freshly hatched larvae may be smaller than 0.05 mm. [more]

At least 1,127 species and subspecies belong to the Phylum Tardigrada.

More info about the Phylum Tardigrada may be found here.

Tentaculata

Tentaculata is a class of comb jellies. The common feature of this class is a pair of long, feathery, contractile tentacles, which can be retracted into specialised ciliated sheaths. In some species, the primary tentacles are reduced and they have smaller, secondary tentacles. The tentacles have colloblasts, which are sticky-tipped cells that trap small prey. [more]

At least 31 species and subspecies belong to the Phylum Tentaculata.

More info about the Phylum Tentaculata may be found here.

Trilobozoa

At least 7 species and subspecies belong to the Phylum Trilobozoa.

More info about the Phylum Trilobozoa may be found here.

Vendobionta

At least 5 species and subspecies belong to the Phylum Vendobionta.

More info about the Phylum Vendobionta may be found here.

Vetulicolia

At least 3 species and subspecies belong to the Phylum Vetulicolia.

More info about the Phylum Vetulicolia may be found here.

References

• 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.

Footnotes

1. ^ "Animal". The American Heritage Dictionary (Forth). (2006). Houghton Mifflin Company. 
2. ^ National Zoo. Panda Classroom (English). Retrieved on September 30, 2007.
3. ^ Jennifer Bergman. Heterotrophs (English). Retrieved on September 30, 2007.
4. ^ Davidson, Michael W.. Animal Cell Structure (English). Retrieved on September 20, 2007.
5. ^ Saupe, S.G. Concepts of Biology (English). Retrieved on September 30, 2007.
6. ^ "Animals More Than 1 Billion Years Ago: Trace Fossil Evidence from India" (1998). Science 282: 80-83. Retrieved on 2007-08-20. 
7. ^ Dunn et al. 2008. "Broad phylogenomic sampling improves resolution of theanimal tree of life". Nature 06614.
8. ^ "Acoel Flatworms: Earliest Extant Bilaterian Metazoans, Not Members of Platyhelminthes" (March 1999). Science 283 (5409): 1919 - 1923. doi:10.1126/science.283.5409.1919. Retrieved on 2007-12-19. 
9. ^ Todaro, Antonio. Gastrotricha: Overview. Gastrotricha: World Portal. University of Modena & Reggio Emilia. Retrieved on 2008-01-26.
10. ^ Kristensen, Reinhardt Møbjerg (July 2002). "An Introduction to Loricifera, Cycliophora, and Micrognathozoa". Integrative and Comparative Biology 42 (3): 641-651. Oxford Journals. doi:10.1093/icb/42.3.641. Retrieved on 2008-01-26. 
11. ^ Biodiversity: Mollusca. The Scottish Association for Marine Science. Retrieved on 2007-11-19.
12. ^ Russell, Bruce J. (Writer), Denning, David (Writer). Branches on the Tree of Life: Annelids [VHS]. BioMEDIA ASSOCIATES.
13. ^ "Annelida and Arthropoda are not sister taxa: A phylogenetic analysis of spiralean metazoan morphology" (1992). Systematic Biology 41 (3): 305-330. Retrieved on 2007-11-19. 
14. ^ "Phylogenetic Relationships of Annelids,Molluscs, and Arthropods Evidenced from Molecules and Morphology" (September, 1996). Journal of Molecular Evolution 43 (3): 207-215. New York: Springer. doi:10.1007/PL00006079. ISSN 0022-2844. Retrieved on 2007-11-19. 
15. ^ [|Collins, Allen G.] (1995), The Lophophore, University of California Museum of Paleontology, <http://www.ucmp.berkeley.edu/glossary/gloss7/lophophore.html>
16. ^ "The new animal phylogeny: Reliability and implications" (April, 25 2000). Proceedings of the National Academy of Sciences 97 (9): 4453-4456. ISSN 0022-2844. PMID 10781043. Retrieved on 2007-11-19. 
17. ^ Passamaneck, Yale J. (2003), "Woods Hole Oceanographic Institution", Molecular Phylogenetics of the Metazoan Clade Lophotrochozoa, pp. 124, <http://handle.dtic.mil/100.2/ADA417356>
18. ^ "Phylogenetic relationships among higher nemertean (Nemertea) taxa inferred from 18S rDNA sequences" (September, 2001). Molecular Phylogenetics and Evolution 20 (3): 327-334. doi:10.1006/mpev.2001.0982. Retrieved on 2007-11-19. 
19. ^ "The mitochondrial genome of the Sipunculid Phascolopsis gouldii supports its association with Annelida rather than Mollusca" (PDF) (February, 2002). Molecular Biology and Evolution 19 (2): 127–137. ISSN 0022-2844. PMID 11801741. Retrieved on 2007-11-19. 
20. ^ Nielsen, Claus (April 2001). "Bryozoa (Ectoprocta: ‘Moss’ Animals)". Encyclopedia of Life Sciences. John Wiley & Sons, Ltd. doi:10.1038/npg.els.0001613. Retrieved on 2008-01-19. 
21. ^ 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. 
22. ^ "Mitochondrial Genome of the Homoscleromorph Oscarella carmela (Porifera, Demospongiae) Reveals Unexpected Complexity in the Common Ancestor of Sponges and Other Animals" (2006-10-27). Molecular Biology and Evolution 24 (2): 363-373. Oxford Journals. doi:10.1093/molbev/msl167. Retrieved on 2008-01-19. 

Sources

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