Parenting or child rearing is the process of promoting and supporting the physical, emotional and intellectual development of a child from infancy to adulthood. Parenting refers to the intricacies of raising a child and not to the biological relationship; the most common caretaker in parenting is the biological parent of the child in question, although others may be an older sibling, a grandparent, a legal guardian, uncle or other family member, or a family friend. Governments and society may have a role in child-rearing. In many cases, orphaned or abandoned children receive parental care from non-parent blood relations. Others may be raised in foster care, or placed in an orphanage. Parenting skills vary, a parent with good parenting skills may be referred to as a good parent. Parenting styles vary by historical time period, race/ethnicity, social class, other social features. Additionally, research has supported that parental history both in terms of attachments of varying quality as well as parental psychopathology in the wake of adverse experiences, can influence parental sensitivity and child outcomes.
Social class, wealth and income have a strong impact on what methods of child rearing parents use. Cultural values play a major role in. However, parenting is always evolving. A family's social class plays a large role in the opportunities and resources that will be available to a child. Working-class children grow up at a disadvantage with the schooling and level of parental attention available compared to middle-class or upper-class. Lower working-class families do not get the kind of networking that the middle and upper classes do through helpful family members and community individuals or groups as well as various professionals or experts. A parenting style is indicative of the overall emotional climate in the home. Developmental psychologist Diana Baumrind identified three main parenting styles in early child development: authoritative and permissive; these parenting styles were expanded to four, to include an uninvolved style. On the one hand, these four styles involve combinations of acceptance and responsiveness, on the other hand, involve demand and control.
Research has found that parenting style is related to a child's subsequent mental health and well-being. In particular, authoritative parenting is positively related to mental health and satisfaction with life, authoritarian parenting is negatively related to these variables. Authoritative parenting Described by Baumrind as the "just right" style, it combines a medium level demands on the child and a medium level responsiveness from the parents. Authoritative parents rely on positive infrequent use of punishment. Parents are more aware of a child's feelings and capabilities and support the development of a child's autonomy within reasonable limits. There is a give-and-take atmosphere involved in parent-child communication and both control and support are balanced. Research shows that this style is more beneficial than the too-hard authoritarian style or the too-soft permissive style. Authoritarian parenting styles Authoritarian parents are rigid and strict. High demands are placed on the child.
Parents who practice authoritarian style parenting have a non-negotiable set of rules and expectations that are enforced and require rigid obedience. When the rules are not followed, punishment is used to promote future obedience. There is no explanation of punishment except that the child is in trouble for breaking a rule; this parenting style is associated with corporal punishment, such as spanking and "Because I said so" is a typical response to a child's question of authority. This type of parenting is seen more in working-class families than in the middle class. In 1983 Diana Baumrind found that children raised in an authoritarian-style home were less cheerful, more moody and more vulnerable to stress. In many cases these children demonstrated passive hostility. Permissive parenting Permissive, or indulgent, parenting is more popular in middle-class than in working-class families. In these settings, a child's freedom and autonomy are valued, parents tend to rely on reasoning and explanation.
Parents are undemanding, so there tends to be little if any punishment or explicit rules in this style of parenting. These parents say that their children are free from external constraints and tend to be responsive to whatever the child wants at the time. Children of permissive parents are happy but sometimes show low levels of self-control and self-reliance because they lack structure at home. Uninvolved parenting An uninvolved or neglectful parenting style is when parents are emotionally or physically absent, they have little to no expectation of the child and have no communication. They have little to no behavioral expectations. If present, they may provide. There is a large gap between parents and children with this parenting style. Children with little or no communication with their own parents tended to be victimized by other children and may themselves be exhibit deviant behavior. Children of uninvolved parents suffer in social competence, academic performance, psychosocial developme
Reptiles are tetrapod animals in the class Reptilia, comprising today's turtles, snakes, lizards and their extinct relatives. The study of these traditional reptile orders combined with that of modern amphibians, is called herpetology; because some reptiles are more related to birds than they are to other reptiles, the traditional groups of "reptiles" listed above do not together constitute a monophyletic grouping or clade. For this reason, many modern scientists prefer to consider the birds part of Reptilia as well, thereby making Reptilia a monophyletic class, including all living Diapsids; the earliest known proto-reptiles originated around 312 million years ago during the Carboniferous period, having evolved from advanced reptiliomorph tetrapods that became adapted to life on dry land. Some early examples include Casineria. In addition to the living reptiles, there are many diverse groups that are now extinct, in some cases due to mass extinction events. In particular, the Cretaceous–Paleogene extinction event wiped out the pterosaurs, plesiosaurs and sauropods, as well as many species of theropods, including troodontids, dromaeosaurids and abelisaurids, along with many Crocodyliformes, squamates.
Modern non-avian reptiles inhabit all the continents except Antarctica, although some birds are found on the periphery of Antarctica. Several living subgroups are recognized: Testudines, 350 species. Reptiles are tetrapod vertebrates, creatures that either have four limbs or, like snakes, are descended from four-limbed ancestors. Unlike amphibians, reptiles do not have an aquatic larval stage. Most reptiles are oviparous, although several species of squamates are viviparous, as were some extinct aquatic clades – the fetus develops within the mother, contained in a placenta rather than an eggshell; as amniotes, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings. Extant reptiles range in size from a tiny gecko, Sphaerodactylus ariasae, which can grow up to 17 mm to the saltwater crocodile, Crocodylus porosus, which can reach 6 m in length and weigh over 1,000 kg.
In the 13th century the category of reptile was recognized in Europe as consisting of a miscellany of egg-laying creatures, including "snakes, various fantastic monsters, assorted amphibians, worms", as recorded by Vincent of Beauvais in his Mirror of Nature. In the 18th century, the reptiles were, from the outset of classification, grouped with the amphibians. Linnaeus, working from species-poor Sweden, where the common adder and grass snake are found hunting in water, included all reptiles and amphibians in class "III – Amphibia" in his Systema Naturæ; the terms "reptile" and "amphibian" were interchangeable, "reptile" being preferred by the French. Josephus Nicolaus Laurenti was the first to formally use the term "Reptilia" for an expanded selection of reptiles and amphibians similar to that of Linnaeus. Today, the two groups are still treated under the same heading as herptiles, it was not until the beginning of the 19th century that it became clear that reptiles and amphibians are, in fact, quite different animals, Pierre André Latreille erected the class Batracia for the latter, dividing the tetrapods into the four familiar classes of reptiles, amphibians and mammals.
The British anatomist Thomas Henry Huxley made Latreille's definition popular and, together with Richard Owen, expanded Reptilia to include the various fossil "antediluvian monsters", including dinosaurs and the mammal-like Dicynodon he helped describe. This was not the only possible classification scheme: In the Hunterian lectures delivered at the Royal College of Surgeons in 1863, Huxley grouped the vertebrates into mammals and ichthyoids, he subsequently proposed the names of Ichthyopsida for the latter two groups. In 1866, Haeckel demonstrated that vertebrates could be divided based on their reproductive strategies, that reptiles and mammals were united by the amniotic egg; the terms "Sauropsida" and "Theropsida" were used again in 1916 by E. S. Goodrich to distinguish between lizards and their relatives on the one hand and mammals and their extinct relatives on the other. Goodrich supported this division by the nature of the hearts and blood vessels in each group, other features, such as the structure of the forebrain.
According to Goodrich, both lineages evolved from an earlier stem group, Protosauria in which he included some animals today considered reptile-like amphibians, as well as early reptiles. In 1956, D. M. S. Watson observed that the first two groups diverged early in reptilian history, so he divided Goodrich's Protosauria between them, he reinterpreted Sauropsida and Theropsida to exclude birds and mammals, respectively. Thus his Sauropsida included Procolophonia, Millerosauria, Squamata, Rhynchocephalia
The gastric-brooding frogs or platypus frogs were a genus of ground-dwelling frogs native to Queensland in eastern Australia. The genus consisted of only two species; the genus was unique because it contained the only two known frog species that incubated the prejuvenile stages of their offspring in the stomach of the mother. The combined ranges of the gastric-brooding frogs comprised less than 2,000 square kilometres. Both species were associated with creek systems in rainforests at elevations of between 350 and 1,400 metres; the causes of the gastric-brooding frogs' extinction are not understood, but habitat loss and degradation and some diseases may have contributed. The assignment of the genus to a taxonomic family is hotly debated; some biologists class them within Myobatrachidae under the subfamily Rheobatrachinae, but others place them in their own family, Rheobatrachidae. Scientists at the University of Newcastle and University of New South Wales announced in March 2013 that the frog would be the subject of a cloning attempt, referred to as the ”Lazarus Project”, to resurrect the species.
Embryos were cloned, the project hopes to produce a living frog. The southern gastric brooding frog has been listed as Extinct by the IUCN because it has not been recorded in the wild since 1981, extensive searches over the last 35 years have failed to locate this species; the genus Rheobatrachus was first described in 1973 by David Liem and since has not undergone any scientific classification changes. It has been placed in a distinct subfamily of Rheobatrachinae. In 2006, D. R. Frost and colleagues found Rheobatrachus, on the basis of molecular evidence, to be the sister taxon of Mixophyes and placed it within Myobatrachidae. Both species of gastric-brooding frogs were different in appearance and behaviour from other Australian frog species, their large protruding eyes and short, blunt snout along with complete webbing and slimy bodies differentiated them from all other Australian frogs. The aquatic behaviour exhibited by both species was only shared with the Dahl's Aquatic Frog and their ability to raise their young in the mother's stomach was unique among all frogs.
The common names, "gastric-brooding frog" and "platypus frog", are used to describe the two species. "Gastric-brooding" describes the unique way the female raised the young and "platypus" describes their aquatic nature. The southern gastric-brooding frog was discovered in 1972 and described in 1973, there is one publication suggesting that the species was discovered in 1914. Rheobatrachus silus was restricted to the Blackall Range and Conondale Ranges in southeast Queensland, north of Brisbane, between elevations of 350 and 800 metres above sea level; the areas of rainforest, wet sclerophyll forest and riverine gallery open forest that it inhabited were limited to less than 1,400 km2. They were recorded in streams in the catchments of the Mary and Mooloolah Rivers. Depending on the source, the last specimen seen in the wild was in 1979 in the Conondale Range, or in 1981 in the Blackall Ranges; the last captive specimen died in 1983. This species is believed to be extinct; the southern gastric-brooding frog was a medium-sized species of dull colouration, with large protruding eyes positioned close together and a short, blunt snout.
Its skin was coated with mucus. The fingers were long, slender and unwebbed and the toes were webbed; the arms and legs were large in comparison to the body. In both species the females were larger than the males; the southern gastric-brooding frog was a dull grey to slate coloured frog that had small patches, both darker and lighter than the background colouration, scattered over dorsal surface. The ventral surface was white or cream with yellow blotches; the arms and legs were yellow underneath. There was a dark stripe; the ventral surface was pale yellow. The toes and fingers were light brown with pale brown flecking; the end of each digit had a small disc and the iris was dark brown. The skin was finely granular and the tympanum was hidden; the male Southern Gastric Brooding Frog was 33 to 41 millimetres in length and the female 44 to 54 millimetres in length. The southern gastric-brooding frog lived in areas of rainforest, wet sclerophyll forest and riverine gallery open forest, they were a predominately aquatic species associated with watercourses and adjacent rock pools and soaks.
Streams that the southern gastric-brooding frog were found in were permanent and only ceased to flow during years of low rainfall. Sites where southern gastric-brooding frogs were found consisted of closed forests with emergent eucalypts, however there was sites where open forest and grassy ground cover were the predominate vegetation. There is no record for this species occurring in cleared riparian habitat. Searches during spring and summer showed that the favored diurnal habitat was at the edge of rock pools, either amongst leaf litter, under or between stones or in rock crevices, they were found under rocks in shallow water. Winter surveys of sites where southern gastric-brooding frogs were common only recovered two specimens, it is assumed that they hibernated during the colder months. Adult males preferred deeper pool
Fertilisation or fertilization known as generative fertilisation, pollination, fecundation and impregnation, is the fusion of gametes to initiate the development of a new individual organism or offspring. This cycle of fertilisation and development of new individuals is called sexual reproduction. During double fertilisation in angiosperms the haploid male gamete combines with two haploid polar nuclei to form a triploid primary endosperm nucleus by the process of vegetative fertilisation. In Antiquity, Aristotle conceived the formation of new individuals through fusion of male and female fluids, with form and function emerging in a mode called by him as epigenetic. In 1784, Spallanzani established the need of interaction between the female's ovum and male's sperm to form a zygote in frogs. In 1827, von Baer observed a therian mammalian egg for the first time. Oscar Hertwig, in Germany, described the fusion of ova from sea urchin; the evolution of fertilisation is related to the origin of meiosis, as both are part of sexual reproduction, originated in eukaryotes.
There are two conflicting theories on how the couple meiosis–fertilisation arose. One is; the other is. The gametes that participate in fertilisation of plants are the pollen, the egg cell. Various families of plants have differing methods. In Bryophyte land plants, fertilisation takes place within the archegonium. In flowering plants a second fertilisation event involves another sperm cell and the central cell, a second female gamete. In flowering plants there are two sperm from each pollen grain. In seed plants, after pollination, a pollen grain germinates, a pollen tube grows and penetrates the ovule through a tiny pore called a micropyle; the sperm are transferred from the pollen through the pollen tube to the ovule. Pollen tube growth Unlike animal sperm, motile, plant sperm is immotile and relies on the pollen tube to carry it to the ovule where the sperm is released; the pollen tube penetrates the stigma and elongates through the extracellular matrix of the style before reaching the ovary.
Near the receptacle, it breaks through the ovule through the micropyle and the pollen tube "bursts" into the embryo sac, releasing sperm. The growth of the pollen tube has been believed to depend on chemical cues from the pistil, however these mechanisms were poorly understood until 1995. Work done on tobacco plants revealed a family of glycoproteins called TTS proteins that enhanced growth of pollen tubes. Pollen tubes in a sugar free pollen germination medium and a medium with purified TTS proteins both grew. However, in the TTS medium, the tubes grew at a rate 3x that of the sugar-free medium. TTS proteins were placed on various locations of semi in vevo pollinated pistils, pollen tubes were observed to extend toward the proteins. Transgenic plants lacking the ability to produce TTS proteins exhibited slower pollen tube growth and reduced fertility. Rupture of pollen tube The rupture of the pollen tube to release sperm in Arabidopsis has been shown to depend on a signal from the female gametophyte.
Specific proteins called FER protein kinases present in the ovule control the production of reactive derivatives of oxygen called reactive oxygen species. ROS levels have been shown via GFP to be at their highest during floral stages when the ovule is the most receptive to pollen tubes, lowest during times of development and following fertilization. High amounts of ROS activate Calcium ion channels in the pollen tube, causing these channels to take up Calcium ions in large amounts; this increased uptake of calcium causes the pollen tube to rupture, release its sperm into the ovule. Pistil feeding assays in which plants were fed diphenyl iodonium chloride suppressed ROS concentrations in Arabidopsis, which in turn prevented pollen tube rupture. Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called "non-vascular plants"; some bryophytes do have specialised tissues for the transport of water. A fern is a member of a group of 12,000 species of vascular plants that reproduce via spores and have neither seeds nor flowers.
They differ from mosses by being vascular. They leaves, like other vascular plants. Most ferns have what are called fiddleheads that expand into fronds, which are each delicately divided; the gymnosperms are a group of seed producing plants that includes conifers, Cycads and Gnetales. The term "gymnosperm" comes from the Greek composite word γυμνόσπερμος, meaning "naked seeds", after the unenclosed condition of their seeds, their naked condition stands in contrast to the seeds and ovules of flowering plants, which are enclosed within an ovary. Gymnosperm seeds develop either on the surface of scales or leaves modified to form cones, or at the end of short stalks as in Ginkgo. After being fertilised, the ovary starts to develop into the fruit. With multi-seeded fruits, multiple grains of pollen are necessary for syngamy with each ovule; the growth of the pollen tube is controlled by the vegetative cytoplasm. Hydrolytic enzymes are secreted by the pollen tube that digest the female tissue as the tube grows down the stigma and style
Pregnancy in fish
Pregnancy has been traditionally defined as the period during which developing embryos are incubated in the body after egg-sperm union. Although the term refers to placental mammals, it has been used in the titles of many international, peer-reviewed, scientific articles on fish, e.g. Consistent with this definition, there are several modes of reproduction in fish, providing different amounts of parental care. In ovoviviparity, there is internal fertilization and the young are born live but there is no placental connection or significant trophic interaction. There are two types of viviparity in fish. In histotrophic viviparity, the zygotes develop in the female's oviducts, but she provides no direct nutrition. In hemotrophic viviparity, the zygotes are retained within the female and are provided with nutrients by her through some form of placenta. In seahorses and pipefish, it is the male. Pregnancy has been traditionally defined as the period during which developing embryos are incubated in the body after egg-sperm union.
Despite strong similarities between viviparity in mammals, researchers have been reluctant to use the term "pregnancy" for non-mammals because of the developed form of viviparity in eutherians. Recent research into physiological and genetic changes associated with fish reproduction provide evidence that incubation in some species is a specialized form of reproduction similar to other forms of viviparity. Although the term "pregnancy" refers to eutherian animals, it has been used in the titles of many international, peer-reviewed, scientific articles on fish, e.g. Five modes of reproduction can be differentiated in fish based on relations between the zygote and parents: Ovuliparity: Fertilization of eggs is external. Oviparity: Fertilization of eggs is internal. Ovoviviparity: Fertilization is internal; the embryos depend upon their yolk for survival. There are two types of viviparity among fish. Histotrophic viviparity: The zygotes develop in the female's oviducts, but she provides no direct nutrition.
The embryos survive by eating their unborn siblings. Hemotrophic viviparity: The zygotes are retained within the female and are provided with nutrients by her through some form of placenta. Examples of ovoviviparous fish are many of the squaliform sharks, which include sand sharks, mackerel sharks, nurse sharks, requiem sharks, dog sharks and hammerheads, among others, the lobe finned coelacanth; some species of rockfish and sculpins produce rather weak larvae with no egg membrane and are by definition, ovoviviparous. Ovoviviparity occurs in most live-bearing bony fishes. Viviparous fish include the families Goodeidae, Jenynsiidae, Poeciliidae and some sharks; the halfbeaks, are found in both marine and freshwaters and those species that are marine produce eggs with extended filaments that attach to floating or stationary debris, while those that are found in freshwater are viviparous with internal fertilization. The Bythitidae are viviparous although one species, Dinematichthys ilucoeteoides, is reported to be ovoviviparous.
Aquarists refer to ovoviviparous and viviparous fish as "livebearers". Examples include guppies, moonfish, four-eyed fish and swordtails. All of these varieties exhibit signs of their pregnancy before the live fry are born; as examples, the female swordtail and guppy will both give birth to anywhere from 20 to 100 live young after a gestation period of four to six weeks, mollies will produce a brood of 20 to 60 live young after a gestation of six to 10 weeks. Other terms relating to pregnancy in fish relate to the differences in the mode and extent of support the female gives the developing offspring. "Lecithotrophy" occurs when the mother provisions the oocyte with all the resources it needs prior to fertilization, so the egg is independent of the mother. Many members of the fish family Poeciliidae are considered to be lecithotrophic, research is showing that others are matrotrophic."Aplacental viviparity" occurs when the female retains the embryos during the entire time of development but without any transfer of nutrients to the young.
The yolk sac is the only source of nutrients for the developing embryo. There are at least two exceptions to this. "Matrotrophy" occurs when the embryo exhausts its yolk supply early in gestation and the mother provides additional nutrition. Post-fertilization transfer of nutrients has been reported in several species within the genera Gambusia and Poecilia G. affinis, G. clarkhubbsi, G. holbrooki, G. gaigei, G. geiseri, G. nobilis, P. formosa, P. latipinna, P. mexicana. Viviparous fish have developed several ways of providing their offspring with nutrition. "Embryotrophic" or "histrotrophic" nutrition occurs by the production of nutritive fluid, uterine milk, by the uterine lining, absorbed directly by the developing embryo. "Hemotrophic" nutrition occu
Birds known as Aves, are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, a strong yet lightweight skeleton. Birds range in size from the 5 cm bee hummingbird to the 2.75 m ostrich. They rank as the world's most numerically-successful class of tetrapods, with ten thousand living species, more than half of these being passerines, sometimes known as perching birds. Birds have wings which are less developed depending on the species. Wings, which evolved from forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in flightless birds, including ratites and diverse endemic island species of birds; the digestive and respiratory systems of birds are uniquely adapted for flight. Some bird species of aquatic environments seabirds and some waterbirds, have further evolved for swimming; the fossil record demonstrates that birds are modern feathered dinosaurs, having evolved from earlier feathered dinosaurs within the theropod group, which are traditionally placed within the saurischian dinosaurs.
The closest living relatives of birds are the crocodilians. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period, around 170 million years ago. Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of powered flight, many retained primitive characteristics like toothy jaws in place of beaks, long bony tails. DNA-based evidence finds that birds diversified around the time of the Cretaceous–Palaeogene extinction event 66 million years ago, which killed off the pterosaurs and all the non-avian dinosaur lineages, but birds those in the southern continents, survived this event and migrated to other parts of the world while diversifying during periods of global cooling. This makes them the sole surviving dinosaurs according to cladistics; some birds corvids and parrots, are among the most intelligent animals. Many species annually migrate great distances. Birds are social, communicating with visual signals and bird songs, participating in such social behaviours as cooperative breeding and hunting and mobbing of predators.
The vast majority of bird species are monogamous for one breeding season at a time, sometimes for years, but for life. Other species have breeding systems that are polygynous or polyandrous. Birds produce offspring by laying eggs, they are laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching; some birds, such as hens, lay eggs when not fertilised, though unfertilised eggs do not produce offspring. Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds being important sources of eggs and feathers. Songbirds and other species are popular as pets. Guano is harvested for use as a fertiliser. Birds prominently figure throughout human culture. About 120–130 species have become extinct due to human activity since the 17th century, hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them.
Recreational birdwatching is an important part of the ecotourism industry. The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae. Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system in use. Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda. Aves and a sister group, the clade Crocodilia, contain the only living representatives of the reptile clade Archosauria. During the late 1990s, Aves was most defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica. However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, is used by many scientists including adherents of the Phylocode system. Gauthier defined Aves to include only the crown group of the set of modern birds; this was done by excluding most groups known only from fossils, assigning them, instead, to the Avialae, in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.
Gauthier identified four different definitions for the same biological name "Aves", a problem. Gauthier proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below, he assigned other names to the other groups. Aves can mean all archosaurs closer to birds than to crocodiles Aves can mean those advanced archosaurs with feathers Aves can mean those feathered dinosaurs that fly Aves can mean the last common ancestor of all the living birds and all of its descendants (a "c
Ctenophora comprise a phylum of invertebrate animals that live in marine waters worldwide. They are notable for the groups of cilia they use for swimming, they are the largest animals to swim with the help of cilia. Depending on the species, adult ctenophores range from a few millimeters to 1.5 m in size. Only 100 to 150 species have been validated, another 25 have not been described and named; the textbook examples are cydippids with egg-shaped bodies and a pair of retractable tentacles fringed with tentilla that are covered with colloblasts, sticky cells that capture prey. Their bodies consist of a mass of jelly, with one layer two cells thick on the outside and another lining the internal cavity; the phylum has a wide range of body forms, including the egg-shaped cydippids with retractable tentacles that capture prey, the flat combless platyctenids, the large-mouthed beroids, which prey on other ctenophores. All ctenophores function as predators, taking prey ranging from microscopic larvae and rotifers to the adults of small crustaceans.
Despite their soft, gelatinous bodies, fossils thought to represent ctenophores appear in lagerstätten dating as far back as the early Cambrian, about 525 million years ago. The position of the ctenophores in the "tree of life" has long been debated in molecular phylogenetics studies. Biologists proposed that ctenophores constitute the second-earliest branching animal lineage, with sponges being the sister-group to all other multicellular animals. Other biologists once believed that ctenophores were emerging earlier than the sponges, which themselves appeared before the split between cnidarians and bilaterians; however reanalysis of the data showed that the computer algorithms used for analysis were misled by the presence of specific ctenophore genes that were markedly different from those of other species. A recent molecular phylogenetics analysis concluded that the common ancestor of all modern ctenophores was cydippid-like, that all the modern groups appeared recently after the Cretaceous–Paleogene extinction event 66 million years ago.
Evidence accumulating since the 1980s indicates that the "cydippids" are not monophyletic, in other words they do not include all and only the descendants of a single common ancestor, because all the other traditional ctenophore groups descend from various cydippids. Among animal phyla, the Ctenophores are more complex than sponges, about as complex as cnidarians, less complex than bilaterians. Unlike sponges, both ctenophores and cnidarians have: cells bound by inter-cell connections and carpet-like basement membranes. Ctenophores are distinguished from all other animals by having colloblasts, which are sticky and adhere to prey, although a few ctenophore species lack them. Like sponges and cnidarians, ctenophores have two main layers of cells that sandwich a middle layer of jelly-like material, called the mesoglea in cnidarians and ctenophores. Hence ctenophores and cnidarians have traditionally been labelled diploblastic, along with sponges. Both ctenophores and cnidarians have a type of muscle that, in more complex animals, arises from the middle cell layer, as a result some recent text books classify ctenophores as triploblastic, while others still regard them as diploblastic.
The comb jellies have more than 80 different cell types, exceeding the numbers from other groups like placozoans, sponges and some deep-branching bilaterians. Ranging from about 1 millimeter to 1.5 meters in size, ctenophores are the largest non-colonial animals that use cilia as their main method of locomotion. Most species have eight strips, called comb rows, that run the length of their bodies and bear comb-like bands of cilia, called "ctenes", stacked along the comb rows so that when the cilia beat, those of each comb touch the comb below; the name "ctenophora" means "comb-bearing", from the Greek κτείς meaning "comb" and the Greek suffix -φορος meaning "carrying". For a phylum with few species, ctenophores have a wide range of body plans. Coastal species need to be tough enough to withstand waves and swirling sediment particles, while some oceanic species are so fragile that it is difficult to capture them intact for study. In addition oceanic species do not preserve well, are known from photographs and from observers' notes.
Hence most attention has until concentrated on three coastal genera – Pleurobrachia and Mnemiopsis. At least two textbooks base their descriptions of ctenophores on the cydippid Pleurobrachia. Since the body of many species is radially symmetrical, the main axis is oral to aboral However, since only two of the canals near the statocyst terminate in anal pores, ctenophores have no mirror-symmetry, although many have rotational symmetry. In other words, if the animal rotates in a half-circle it looks the same as; the Ctenophore phylum has a wide range of body forms, including the flattened, deep-sea platyctenids, in which the adults of most species lack combs, the coastal beroids, which lack tentacles and prey on other ctenophores by using huge mouths armed with groups of large, stiffened cilia that act as teeth. Like thos