Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, 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 third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
Lophotrochozoa is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence; the Lophotrochozoa has basal Cycliophora and Mollusca groups, more derived Lophophorate and Annelida groups. With the introduction of Platytrochozoa and Rouphozoa, one candidate phylogeny is pictured below – though other studies recover a range of alternative possibilities: A number of fossil taxa can be identified as early Lophotrochozoans if their precise affinity remains contested; the clade Lophotrochozoa is named after the two distinct characteristics of its members, the feeding structure lophophore, a ciliated crown of tentacles surrounding a mouth, the developmental stage trochophore larvae. Lophophorata such as Brachiozoa and Bryozoa have lophophores, while members of Trochozoa such as molluscs and annelids have trochophores. While some may have none. Podsiadlowski, R. "Phylogeny and mitochondrial gene order variation in Lophotrochozoa in the light of new mitogenomic data from Nemertea".
BMC Genomics. 10: 364. Doi:10.1186/1471-2164-10-364. PMC 2728741. PMID 19660126 //www.ncbi.nlm.nih.gov/pmc/articles/PMC2728741
The gastrotrichs referred to as hairybacks, are a group of microscopic, worm-like, pseudocoelomate animals, are distributed and abundant in freshwater and marine environments. They are benthic and live within the periphyton, the layer of tiny organisms and detritus, found on the seabed and the beds of other water bodies; the majority live on and between particles of sediment or on other submerged surfaces, but a few species are terrestrial and live on land in the film of water surrounding grains of soil. Gastrotrichs are divided into two orders, the Macrodasyida which are marine, the Chaetonotida, some of which are marine and some freshwater. Nearly eight hundred species of gastrotrich have been described. Gastrotrichs have a simple body plan with a head region, with a brain and sensory organs, a trunk with a simple gut and the reproductive organs, they have adhesive glands with which they can anchor themselves to the substrate and cilia with which they move around. They feed on detritus, they are hermaphrodites, the marine species producing eggs which develop directly into miniature adults.
The freshwater species are parthenogenetic, producing unfertilised eggs, at least one species is viviparous. Gastrotrichs have lifespans of only a few days; the name "gastrotrich" comes from the Greek γαστήρ gaster, meaning "stomach", θρίξ thrix, meaning "hair". The name was coined by the Russian zoologist Élie Metchnikoff in 1865; the common name "hairyback" arises from a mistranslation of "gastrotrich". The relationship of gastrotrichs to other phyla is unclear. Morphology suggests that they are close to the Rotifera, or the Nematoda. On the other hand, genetic studies place them as close relatives of the Platyhelminthes, the Ecdysozoa or the Lophotrochozoa; as of 2011, around 790 species have been described. The phylum contains a single class, divided into two orders: the Chaetonotida. Edward Ruppert et al. report that the Macrodasyida are wholly marine, but two rare and poorly known species, Marinellina flagellata and Redudasys fornerise, are known from fresh water. The Chaetonotida comprises both freshwater species.
Gastrotrichs vary in size from about 0.06 to 3 mm in body length. They are bilaterally symmetrical, with a transparent strap-shaped or bowling pin-shaped body, arched dorsally and flattened ventrally; the anterior end is not defined as a head but contains the sense organs and pharynx. Cilia are found on the ventral surface of the head and body; the trunk contains the reproductive organs. At the posterior end of the body are two projections with cement glands; this is a double-gland system where one gland secretes the glue and another secretes a de-adhesive agent to sever the connection. In the Macrodasyida, there are additional adhesive glands at the anterior end and on the sides of the body; the body wall consists of a cuticle, an epidermis and longitudinal and circular bands of muscle fibres. In some primitive species, each epidermal cell has a single cilium, a feature shared only by the gnathostomulans; the whole ventral surface of the animal may be ciliated or the cilia may be arranged in rows, patches or transverse bands.
The cuticle is locally thickened in some gastrotrichs and forms scales and spines. There is no coelom and the interior of the animal is filled with poorly differentiated connective tissue. In the macrodasyidans, Y-shaped cells, each containing a vacuole, surround the gut and may function as a hydrostatic skeleton; the mouth is at the anterior end and opens into an elongated muscular pharynx with a triangular or Y-shaped lumen, lined by myoepithelial cells. The pharynx opens into a cylindrical intestine, lined with glandular and digestive cells; the anus is located on the ventral surface close to the posterior of the body. In some species, there are pores in the pharynx opening to the ventral surface. In the chaetonotidans, the excretory system consists of a single pair of protonephridia, which open through separate pores on the lateral underside of the animal in the midsection of the body. In the macrodasyidans, there are several pairs of these opening along the side of the body. Nitrogenous waste is excreted through the body wall, as part of respiration, the protonephridia are believed to function in osmoregulation.
Unusually, the protonephridia do not take the form of flame cells, instead, the excretory cells consist of a skirt surrounding a series of cytoplasmic rods that in turn enclose a central flagellum. These cells, termed cyrtocytes, connect to a single outlet cell which passes the excreted material into the protonephridial duct; as is typical for such small animals, there are circulatory organs. The nervous system is simple; the brain consists of one on either side of the pharynx, connected by a commissure. From these lead a pair of nerve cords which run along either side of the body beside the longitudinal muscle bands; the primary sensory organs are the bristles and ciliated tufts of the body surface which function as mechanoreceptors. There are ciliated pits on the head, simple ciliary photoreceptors and fleshy appendages which act as chemoreceptors. Gastrotrichs are cosmopolitan in distribution, they inhabit the interstitial spaces between particles in marine and freshwater environments, the surfaces of aquatic plants and other submerged objects and the surface film of water surrounding soil particles on
The paraphyletic "Platyzoa" are a group of protostome unsegmented animals proposed by Thomas Cavalier-Smith in 1998. Cavalier-Smith included in Platyzoa the phylum Platyhelminthes, a new phylum, the Acanthognatha, into which he gathered several described phyla of microscopic animals. More it has been described as paraphyletic, containing the Rouphozoa and the Gnathifera. One scheme placed the following phyla in Platyzoa: RouphozoaPlatyhelminthes Gastrotricha GnathiferaSyndermata Rotifera Seisonida Acanthocephala Gnathostomulida Micrognathozoa Cycliophora None of the Platyzoa groups have a respiration or circulation system because of their small size, flat body or parasitic lifestyle; the Platyhelminthes and Gastrotricha are acoelomate. The other phyla have a pseudocoel, share characteristics such as the structure of their jaws and pharynx, although these have been secondarily lost in the parasitic Acanthocephala, they form. The name "Platyzoa" is used; the Platyzoa are close relatives of the Lophotrochozoa.
Together the two make up the Spiralia. Syndermata was a proposed clade that included Acanthocephala and rotifers, but as it appears they are not sister groups after all, the clade has been abandoned. A recent possible cladogram is shown which would show that the Lophotrochozoa emerged within Platyzoa as a sister group of the Rouphozoa; the Lophotrochozoa and Rouphozoa are named the Platytrochozoa. This makes the Platyzoa a paraphyletic group; the Taxonomicon - Taxon: Infrakingdom Platyzoa Cavalier-Smith, 1998 - retrieved January 31, 2006 Triploblastic Relationships with Emphasis on the Acoelomates and the Position of Gnathostomulida, Cycliophora and Chaetognatha: A Combined Approach of 18S rDNA Sequences and Morphology - retrieved January 31, 2006 Myzostomida Are Not Annelids: Molecular and Morphological Support for a Clade of Animals with Anterior Sperm Flagella - retrieved January 31, 2006 Current advances in the phylogenetic reconstruction of metazoan evolution. A new paradigm for the Cambrian explosion?
- retrieved January 31, 2006
Brachiozoa is a grouping of lophophorate animals including Brachiopoda and Phoronida
Nemertea is a phylum of invertebrate animals known as "ribbon worms" or "proboscis worms". Alternative names for the phylum have included Nemertini and Rhynchocoela.. Most are slim only a few millimeters wide, although a few have short but wide bodies. Many have patterns of yellow, orange and green coloration; the foregut and intestine run a little below the midline of the body, the anus is at the tip of the tail, the mouth is under the front. A little above the gut is the rhynchocoel, a cavity which runs above the midline and ends a little short of the rear of the body. All species have a proboscis which lies in the rhynchocoel when inactive but everts to emerge just above the mouth and capture the animal's prey with venom. A extensible muscle in the back of the rhynchocoel pulls the proboscis in when an attack ends. A few species with stubby bodies filter feed and have suckers at the front and back ends, with which they attach to a host; the brain is a ring of four ganglia, positioned around the rhynchocoel near its front end.
At least a pair of ventral nerve cords run along the length of the body. Most nemerteans have various chemoreceptors, on their heads some species have a number of pigment-cup ocelli; these ocelli can not form an image. Nemerteans respire through the skin, they have at least two lateral vessels which are joined at the ends to form a loop, these and the rhynchocoel are filled with fluid. There is no heart, the flow of fluid depends on contraction of muscles in the vessels and the body wall. To filter out soluble waste products, flame cells are embedded in the front part of the two lateral fluid vessels, remove the wastes through a network of pipes to the outside. All nemerteans move using their external cilia to glide on surfaces on a trail of slime, while larger species use muscular waves to crawl, some swim by dorso-ventral undulations. A few live in the open ocean while the rest make hiding places on the bottom. About a dozen species inhabit freshwater in the tropics and subtropics, another dozen species live on land in cool, damp places.
Most nemerteans are carnivores, feeding on annelids and crustaceans. A few species are scavengers, a few species live commensally inside the mantle cavity of molluscs; some species have devastated commercial fishing of crabs. Nemerteans have few predators. Two species are sold as fish bait. In most species the sexes are separate. Nemerteans have numerous temporary gonads, build temporary gonoducts, one per gonad, when the ova and sperm are ready; the eggs are fertilised externally. Some species shed them into the water, others protect their eggs in various ways; the fertilized egg divides by spiral cleavage and grows by determinate development, in which the fate of a cell can be predicted from its predecessors in the process of division. The embryos of most taxa develop either directly to form juveniles or to form planuliform larvae, in which the larva's long axis is the same as the juvenile's. However, some form a pilidium larva, in which the developing juvenile has a gut which lies across the larva's body, eats the remains of the larva when it emerges.
The bodies of some species fragment and parts near the tail can grow full bodies. It has been suggested that three fossil species may be nemerteans. Traditional taxonomy divides the phylum in two classes, Anopla with two orders, Enopla with two orders. However, it is now accepted that Anopla are paraphyletic, as one order is more related to Enopla than to the other order of Anopla; the phylum Nemertea is monophyletic. Its synapomorphies include the eversible proboscis. Traditional taxonomy says that nemerteans are related to flatworms. Both phyla are regarded as members of the Lophotrochozoa, a large "super-phylum" that includes molluscs, brachiopods and many other protostomes. In 1555 Olaus Magnus wrote of a marine worm, 17.76 metres long, about the width of a child's arm, whose touch made a hand swell. William Borlase wrote in 1758 of a "sea long worm", in 1770 Gunnerus wrote a formal description of this animal, which he called Ascaris longissima, its current name, Lineus longissimus, was first used in 1806 by Sowerby.
In 1995, a total of 1,149 species had been grouped into 250 genera. Nemertea are named after one of the daughters of Nereus and Doris. Alternative names for the phylum have included Nemertini and Rhynchocoela; the Nemertodermatida are a separate phylum. Nemerteans are unusual animals; the typical nemertean body is slim in proportion to its length. The smallest are a few millimeters long, most are less than 20 centimetres, several exceed 1 metre; the longest animal found, at 54 metres long, may be a specimen of Lineus longissimus, although L. longissimus is only a few millimeters wide. The bodies of most nemerteans can stretch a lot, up to 10 times their resting length in some species, but reduce their length to 50% and increase their width to 300% when disturbed. A few have short but wide bodies, for example Malacobdella grossa is up to 3.5 centimetres long and 1 centimetre wide, some of these are much less stret
The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, other forms were described by Antonie van Leeuwenhoek in 1703. Most rotifers are around 0.1–0.5 mm long, are common in freshwater environments throughout the world with a few saltwater species. Some rotifers are free swimming and planktonic, others move by inchworming along a substrate, some are sessile, living inside tubes or gelatinous holdfasts that are attached to a substrate. About 25 species are either sessile or planktonic. Rotifers are an important part of the freshwater zooplankton, being a major foodsource and with many species contributing to the decomposition of soil organic matter. Most species of the rotifers are cosmopolitan, but there are some endemic species, like Cephalodella vittata to Lake Baikal. Recent barcoding evidence, suggests that some'cosmopolitan' species, such as Brachionus plicatilis, B. calyciflorus, Lecane bulla, among others, are species complexes.
In some recent treatments, rotifers are placed with acanthocephalans in a larger clade called Syndermata. Rev. John Harris first described the rotifers in 1696 as "an animal like a large maggot which could contract itself into a spherical figure and stretch itself out again. In 1702, Antonie van Leeuwenhoek gave a detailed description of Rotifer vulgaris and subsequently described Melicerta ringens and other species, he was the first to publish observations of the revivification of certain species after drying. Other forms were described by other observers, but it wasn't until the publication of Christian Gottfried Ehrenberg's Die Infusionsthierchen als vollkommene Organismen in 1838 that the rotifers were recognized as being multicellular animals. About 2200 species of rotifers have been described, their taxonomy is in a state of flux. One treatment places them in the phylum Rotifera, with three classes: Seisonidea and Monogononta; the largest group is the Monogononta, with about 1500 species, followed by the Bdelloidea, with about 350 species.
There are only two known genera with three species of Seisonidea. The Acanthocephala considered to be a separate phylum, have been demonstrated to be modified rotifers; the exact relationship to other members of the phylum has not yet been resolved. One possibility is that the Acanthocephala are closer to the Bdelloidea and Monogononta than to the Seisonidea; the Rotifera speaking, are confined to the Bdelloidea and the Monogononta. Rotifera and Seisonida make up a clade called Syndermata; the word "rotifer" is derived from a Latin word meaning "wheel-bearer", due to the corona around the mouth that in concerted sequential motion resembles a wheel. Rotifers have a variety of different shapes; the body of a rotifer is divided into a head and foot, is somewhat cylindrical. There is a well-developed cuticle, which may be thick and rigid, giving the animal a box-like shape, or flexible, giving the animal a worm-like shape. Rigid cuticles are composed of multiple plates, may bear spines, ridges, or other ornamentation.
Their cuticle is formed from sclerotized proteins. The most distinctive feature of rotifers is the presence of a ciliated structure, called the corona, on the head. In the more primitive species, this forms a simple ring of cilia around the mouth from which an additional band of cilia stretches over the back of the head. In the great majority of rotifers, this has evolved into a more complex structure. Modifications to the basic plan of the corona include alteration of the cilia into bristles or large tufts, either expansion or loss of the ciliated band around the head. In genera such as Collotheca, the corona is modified to form a funnel surrounding the mouth. In many species, such as Testudinella, the cilia around the mouth have disappeared, leaving just two small circular bands on the head. In the bdelloids, this plan is further modified, with the upper band splitting into two rotating wheels, raised up on a pedestal projecting from the upper surface of the head; the trunk forms the major part of the body, encloses most of the internal organs.
The foot projects from the rear of the trunk, is much narrower, giving the appearance of a tail. The cuticle over the foot forms rings, making it appear segmented, although the internal structure is uniform. Many rotifers can retract the foot or wholly into the trunk; the foot ends in from one to four toes, which, in sessile and crawling species, contain adhesive glands to attach the animal to the substratum. In many free-swimming species, the foot as a whole is reduced in size, may be absent; the coronal cilia create a current. The mouth opens into a characteristic chewing pharynx, sometimes via a ciliated tube, sometimes directly; the pharynx has a powerful muscular wall and contains tiny, jaw-like structures called trophi, which are the only fossilizable parts of a rotifer. The shape of the trophi varies between different species, depending on the nature of their diet. In suspension feeders, the trophi are covered in grinding ridges, while in more carnivorous species, they may be shaped like forceps to help bite into prey.
In some e