Vertebrates comprise all species of animals within the subphylum Vertebrata. Vertebrates represent the overwhelming majority of the phylum Chordata, with about 69,276 species described. Vertebrates include the jawless fishes and jawed vertebrates, which include the cartilaginous fishes and the bony fishes; the bony fishes in turn, cladistically speaking include the tetrapods, which include amphibians, reptiles and mammals. Extant vertebrates range in size from the frog species Paedophryne amauensis, at as little as 7.7 mm, to the blue whale, at up to 33 m. Vertebrates make up less than five percent of all described animal species; the vertebrates traditionally include the hagfish, which do not have proper vertebrae due to their loss in evolution, though their closest living relatives, the lampreys, do. Hagfish do, possess a cranium. For this reason, the vertebrate subphylum is sometimes referred to as "Craniata" when discussing morphology. Molecular analysis since 1992 has suggested that hagfish are most related to lampreys, so are vertebrates in a monophyletic sense.
Others consider them a sister group of vertebrates in the common taxon of craniata. The word vertebrate derives from the Latin word vertebratus. Vertebrate is derived from the word vertebra, which refers to any of the bones or segments of the spinal column. All vertebrates are built along the basic chordate body plan: a stiff rod running through the length of the animal, with a hollow tube of nervous tissue above it and the gastrointestinal tract below. In all vertebrates, the mouth is found at, or right below, the anterior end of the animal, while the anus opens to the exterior before the end of the body; the remaining part of the body continuing after the anus forms a tail with vertebrae and spinal cord, but no gut. The defining characteristic of a vertebrate is the vertebral column, in which the notochord found in all chordates has been replaced by a segmented series of stiffer elements separated by mobile joints. However, a few vertebrates have secondarily lost this anatomy, retaining the notochord into adulthood, such as the sturgeon and coelacanth.
Jawed vertebrates are typified by paired appendages, but this trait is not required in order for an animal to be a vertebrate. All basal vertebrates breathe with gills; the gills are carried right behind the head, bordering the posterior margins of a series of openings from the pharynx to the exterior. Each gill is supported by a cartilagenous or bony gill arch; the bony fish have three pairs of arches, cartilaginous fish have five to seven pairs, while the primitive jawless fish have seven. The vertebrate ancestor no doubt had more arches than this, as some of their chordate relatives have more than 50 pairs of gills. In amphibians and some primitive bony fishes, the larvae bear external gills, branching off from the gill arches; these are reduced in adulthood, their function taken over by the gills proper in fishes and by lungs in most amphibians. Some amphibians retain the external larval gills in adulthood, the complex internal gill system as seen in fish being irrevocably lost early in the evolution of tetrapods.
While the more derived vertebrates lack gills, the gill arches form during fetal development, form the basis of essential structures such as jaws, the thyroid gland, the larynx, the columella and, in mammals, the malleus and incus. The central nervous system of vertebrates is based on a hollow nerve cord running along the length of the animal. Of particular importance and unique to vertebrates is the presence of neural crest cells; these are progenitors of stem cells, critical to coordinating the functions of cellular components. Neural crest cells migrate through the body from the nerve cord during development, initiate the formation of neural ganglia and structures such as the jaws and skull; the vertebrates are the only chordate group to exhibit cephalisation, the concentration of brain functions in the head. A slight swelling of the anterior end of the nerve cord is found in the lancelet, a chordate, though it lacks the eyes and other complex sense organs comparable to those of vertebrates.
Other chordates do not show any trends towards cephalisation. A peripheral nervous system branches out from the nerve cord to innervate the various systems; the front end of the nerve tube is expanded by a thickening of the walls and expansion of the central canal of spinal cord into three primary brain vesicles: The prosencephalon and rhombencephalon, further differentiated in the various vertebrate groups. Two laterally placed eyes form around outgrowths from the midbrain, except in hagfish, though this may be a secondary loss; the forebrain is well developed and subdivided in most tetrapods, while the midbrain dominates in many fish and some salamanders. Vesicles of the forebrain are paired, giving rise to hemispheres like the cerebral hemispheres in mammals; the resulting anatomy of the central nervous system, with a single hollow nerve cord topped by a series of vesicles, is unique to vertebrates. All invertebrates with well-developed brains, such as insects and squids, have a ventral rather than dorsal system of ganglions, with a split brain stem running on each side of the mouth or gut.
Vertebrates originated about 525 million years ago during the Cambrian explosion, which saw
An embryo is an early stage of development of a multicellular diploid eukaryotic organism. In general, in organisms that reproduce sexually, an embryo develops from a zygote, the single cell resulting from the fertilization of the female egg cell by the male sperm cell; the zygote possesses half the DNA from each of its two parents. In plants and some protists, the zygote will begin to divide by mitosis to produce a multicellular organism; the result of this process is an embryo. In human pregnancy, a developing fetus is considered as an embryo until the ninth week, fertilization age, or eleventh-week gestational age. After this time the embryo is referred to as a fetus. First attested in English in the mid-14c; the word embryon itself from Greek ἔμβρυον, lit. "young one", the neuter of ἔμβρυος, lit. "growing in", from ἐν, "in" and βρύω, "swell, be full". In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of blastula and organogenesis; the blastula stage features a fluid-filled cavity, the blastocoel, surrounded by a sphere or sheet of cells called blastomeres.
In a placental mammal, an ovum is fertilized in a fallopian tube through which it travels into the uterus. An embryo is called a fetus at a more advanced stage of development and up until hatching. In humans, this is from the eleventh week of gestation. However, animals which develop in eggs outside the mother's body, are referred to as embryos throughout development. During gastrulation the cells of the blastula undergo coordinated processes of cell division, and/or migration to form two or three tissue layers. In triploblastic organisms, the three germ layers are called endoderm and mesoderm; the position and arrangement of the germ layers are species-specific, depending on the type of embryo produced. In vertebrates, a special population of embryonic cells called the neural crest has been proposed as a "fourth germ layer", is thought to have been an important novelty in the evolution of head structures. During organogenesis and cellular interactions between germ layers, combined with the cells' developmental potential, or competence to respond, prompt the further differentiation of organ-specific cell types.
For example, in neurogenesis, a subpopulation of ectoderm cells is set aside to become the brain, spinal cord, peripheral nerves. Modern developmental biology is extensively probing the molecular basis for every type of organogenesis, including angiogenesis, myogenesis and many others. In botany, a seed plant embryo is part of a seed, consisting of precursor tissues for the leaves and root, as well as one or more cotyledons. Once the embryo begins to germinate—grow out from the seed—it is called a seedling. Bryophytes and ferns produce an embryo, but do not produce seeds. In these plants, the embryo begins its existence attached to the inside of the archegonium on a parental gametophyte from which the egg cell was generated; the inner wall of the archegonium lies in close contact with the "foot" of the developing embryo. The structure and development of the rest of the embryo varies by group of plants; as the embryo has expanded beyond the enclosing archegonium, it is no longer termed an embryo.
Embryos are used in various fields of research and in techniques of assisted reproductive technology. An egg may be fertilized in vitro and the resulting embryo may be frozen for use; the potential of embryonic stem cell research, reproductive cloning, germline engineering are being explored. Prenatal diagnosis or preimplantation diagnosis enables testing embryos for conditions. Cryoconservation of animal genetic resources is a practice in which animal germplasms, such as embryos are collected and stored at low temperatures with the intent of conserving the genetic material; the embryos of Arabidopsis thaliana have been used as a model to understand gene activation and organogenesis of seed plants. In regards to research using human embryos, the ethics and legalities of this application continue to be debated. Researchers from MERLN Institute and the Hubrecht Institute in the Netherlands managed to grow samples of synthetic rodent embryos, combining certain types of stem cells; this method will help scientists to more study the first moments of the process of the birth of a new life, which, in turn, can lead to the emergence of new effective methods to combat infertility and other genetic diseases.
Fossilized animal embryos are known from the Precambrian, are found in great numbers during the Cambrian period. Fossilized dinosaur embryos have been discovered; some embryos do not survive to the next stage of development. When this happens it is called spontaneous abortion or miscarriage. There are many reasons; the most common natural cause of miscarriage is chromosomal abnormality in animals or genetic load in plants. In species which produce multiple embryos at the same time, miscarriage or abortion of some embryos can provide the remaining embryos with a greater share of maternal resources; this can disturb the pregnancy, causing harm to the second embryo. Genetic strains which miscarry their embryos are the source of commercial seedl
The supraesophageal ganglion is the first part of the arthropod and insect central nervous system. It receives and processes information from the first and third metameres; the supraesophageal ganglion lies dorsal to the esophagus and consists of three parts, each a pair of ganglia that may be more or less pronounced, reduced, or fused depending on the genus: The protocerebrum, associated with the eyes. Directly associated with the eyes is the optic lobe, as the visual center of the brain; the deutocerebrum processes sensory information from the antennae. It consists of two parts, the antennal lobe that receives axons of olfactory receptor neurons from the antenna and the dorsal lobe that receives mechanosensory and gustatory receptor neurons from the antenna; the dorsal lobe contains motor neurons which controls the antennal muscles. Chelicerata with their missing antennae have a reduced deutocerebrum; the tritocerebrum integrates sensory inputs from the previous two pairs of ganglia. The lobes of the tritocerebrum split to circumvent the esophagus and begin the subesophageal ganglion.
The subesophageal ganglion lies ventral to the esophagus. The segmental ganglia of the ventral nerve cord are found in each body segment as a fused ganglion and they provide the segments with some autonomous control. Lateral horn of insect brain Mushroom bodies Virtual Fly Brain Drosophila connectome Erber, J.. "Visual interneurons in the median protocerebrum of the bee". Journal of Comparative Physiology. 121: 65–77. Doi:10.1007/bf00614181. Wong, Allan M. Jing W. Wang, Richard Axel. "Spatial Representation of the Glomerular Map in the Drosophila Protocerebrum". Cell. 109: 229–241. Doi:10.1016/S0092-867400707-9. PMID 12007409. CS1 maint: Multiple names: authors list Malun, D.. "Connections between the deutocerebrum and the protocerebrum, neuroanatomy of several classes of deutocerebral projection neurons in the brain of male Periplaneta americana". J. Comp. Neurol. 329: 143–162. Doi:10.1002/cne.903290202. PMID 8454728. Flanagan, Daniel. "Morphology and response characteristics of neurones in the deutocerebrum of the brain in the honeybeeApis mellifera".
Journal of Comparative Physiology A. 164: 483–494. Doi:10.1007/bf00610442. Childress, Steven A.. McIver, Susan. "Morphology of the deutocerebrum of female Aedes aegypti". Canadian Journal of Zoology. 62: 1320–1328. Doi:10.1139/z84-190. Technau, Gerhard. Brain development in Drosophila melanogaster. New York Austin, Tex: Springer Science+Business Media Landes Bioscience. Doi:10.1007/978-0-387-78261-4. ISBN 978-0-387-78260-7. OCLC 314349837. Aubele and Nikolai Klemm. "Origin and mapping of tritocerebral neurons of locust". Cell and Tissue Research. 178: 199–219. Doi:10.1007/bf00219048. PMID 66098. CS1 maint: Multiple names: authors list Chaudonneret, J. "Evolution of the insect brain with special reference to the so-called tritocerebrum." Arthropod brain. Wiley, New York: 3-26. "Behavioral Neuroscience, lecture on Honey Bee and its behavior - Neuroanatomy". Retrieved 2019-02-08. "Suboesophageal Ganglion". Flybrain
The spinal cord is a long, tubular structure made up of nervous tissue, that extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. It encloses the central canal of the spinal cord; the brain and spinal cord together make up the central nervous system. In humans, the spinal cord begins at the occipital bone where it passes through the foramen magnum, meets and enters the spinal canal at the beginning of the cervical vertebrae; the spinal cord extends down to between the second lumbar vertebrae where it ends. The enclosing bony vertebral column protects the shorter spinal cord, it is around 45 cm in men and around 43 cm long in women. The spinal cord has a varying width, ranging from 13 mm thick in the cervical and lumbar regions to 6.4 mm thick in the thoracic area. The spinal cord functions in the transmission of nerve signals from the motor cortex to the body, from the afferent fibers of the sensory neurons to the sensory cortex, it is a center for coordinating many reflexes and contains reflex arcs that can independently control reflexes.
It is the location of groups of spinal interneurons that make up the neural circuits known as central pattern generators. These circuits are responsible for controlling motor instructions for rhythmic movements such as walking; the spinal cord is the main pathway for information connecting the brain and peripheral nervous system. Much shorter than its protecting spinal column, the human spinal cord originates in the brainstem, passes through the foramen magnum, continues through to the conus medullaris near the second lumbar vertebra before terminating in a fibrous extension known as the filum terminale, it is about 45 cm long in men and around 43 cm in women, ovoid-shaped, is enlarged in the cervical and lumbar regions. The cervical enlargement, stretching from the C5 to T1 vertebrae, is where sensory input comes from and motor output goes to the arms and trunk; the lumbar enlargement, located between L1 and S3, handles sensory input and motor output coming from and going to the legs. The spinal cord is continuous with the caudal portion of the medulla, running from the base of the skull to the body of the first lumbar vertebra.
It does not run the full length of the vertebral column in adults. It is made of 31 segments from which branch one pair of sensory nerve roots and one pair of motor nerve roots; the nerve roots merge into bilaterally symmetrical pairs of spinal nerves. The peripheral nervous system is made up of these spinal roots and ganglia; the dorsal roots are afferent fascicles, receiving sensory information from the skin and visceral organs to be relayed to the brain. The roots terminate in dorsal root ganglia, which are composed of the cell bodies of the corresponding neurons. Ventral roots consist of efferent fibers that arise from motor neurons whose cell bodies are found in the ventral gray horns of the spinal cord; the spinal cord are protected by three layers of tissue or membranes called meninges, that surround the canal. The dura mater is the outermost layer, it forms a tough protective coating. Between the dura mater and the surrounding bone of the vertebrae is a space called the epidural space; the epidural space is filled with adipose tissue, it contains a network of blood vessels.
The arachnoid mater, the middle protective layer, is named for its spiderweb-like appearance. The space between the arachnoid and the underlying pia mater is called the subarachnoid space; the subarachnoid space contains cerebrospinal fluid, which can be sampled with a lumbar puncture, or "spinal tap" procedure. The delicate pia mater, the innermost protective layer, is associated with the surface of the spinal cord; the cord is stabilized within the dura mater by the connecting denticulate ligaments, which extend from the enveloping pia mater laterally between the dorsal and ventral roots. The dural sac ends at the vertebral level of the second sacral vertebra. In cross-section, the peripheral region of the cord contains neuronal white matter tracts containing sensory and motor axons. Internal to this peripheral region is the grey matter, which contains the nerve cell bodies arranged in the three grey columns that give the region its butterfly-shape; this central region surrounds the central canal, an extension of the fourth ventricle and contains cerebrospinal fluid.
The spinal cord is elliptical in cross section, being compressed dorsolaterally. Two prominent grooves, or sulci, run along its length; the posterior median sulcus is the groove in the dorsal side, the anterior median fissure is the groove in the ventral side. The human spinal cord is divided into segments. Six to eight motor nerve rootlets branch out of right and left ventro lateral sulci in a orderly manner. Nerve rootlets combine to form nerve roots. Sensory nerve rootlets form off right and left dorsal lateral sulci and form sensory nerve roots; the ventral and dorsal roots combine to form one on each side of the spinal cord. Spinal nerves, with the exception of C1 and C2, form inside the intervertebral foramen; these rootlets form the demarcation between the peripheral nervous systems. The grey column, in the center of the cord, is shaped like a butterfly and consists of cell bodies of interneurons, motor neurons, neuroglia cells and unmyelinated axons; the anterior and posterior grey column present as projections of the grey matter and are known as the horns of the spinal cord.
Together, the gr
The chicken is a type of domesticated fowl, a subspecies of the red junglefowl. It is one of the most common and widespread domestic animals, with a total population of more than 19 billion as of 2011. There are more chickens in the world than domesticated fowl. Humans keep chickens as a source of food and, less as pets. Raised for cockfighting or for special ceremonies, chickens were not kept for food until the Hellenistic period. Genetic studies have pointed to multiple maternal origins in South Asia, Southeast Asia, East Asia, but with the clade found in the Americas, the Middle East and Africa originating in the Indian subcontinent. From ancient India, the domesticated chicken spread to Lydia in western Asia Minor, to Greece by the 5th century BC. Fowl had been known in Egypt since the mid-15th century BC, with the "bird that gives birth every day" having come to Egypt from the land between Syria and Shinar, according to the annals of Thutmose III; the chicken used for regular egg and meat production worldwide are corn-ply Broilers with white feathers, yellowish skin and faster growth rate invented in United States of America In the UK and Ireland, adult male chickens over the age of one year are known as cocks, whereas in the United States, Canada and New Zealand, they are more called roosters.
Males less than a year old are cockerels. Castrated roosters are called capons. Females over a year old are known as hens, younger females as pullets, although in the egg-laying industry, a pullet becomes a hen when she begins to lay eggs, at 16 to 20 weeks of age. In Australia and New Zealand, there is a generic term chook to describe all both sexes; the young are called chicks. "Chicken" referred to young domestic fowl. The species as a whole was called domestic fowl, or just fowl; this use of "chicken" survives in the phrase "Hen and Chickens", sometimes used as a British public house or theatre name, to name groups of one large and many small rocks or islands in the sea. The word "chicken" is sometimes erroneously construed to mean females despite the term "hen" for females being in wide circulation, the term “rooster” for males being that most used. In the Deep South of the United States, chickens are referred to by the slang term yardbird. Chickens are omnivores. In the wild, they scratch at the soil to search for seeds and animals as large as lizards, small snakes, or young mice.
The average chicken may live depending on the breed. The world's oldest known chicken was a hen which died of heart failure at the age of 16 years according to the Guinness World Records. Roosters can be differentiated from hens by their striking plumage of long flowing tails and shiny, pointed feathers on their necks and backs, which are of brighter, bolder colours than those of females of the same breed. However, in some breeds, such as the Sebright chicken, the rooster has only pointed neck feathers, the same colour as the hen's; the identification can be made by looking at the comb, or from the development of spurs on the male's legs. Adult chickens have a fleshy crest on their heads called a comb, or cockscomb, hanging flaps of skin either side under their beaks called wattles. Collectively and other fleshy protuberances on the head and throat are called caruncles. Both the adult male and female have wattles and combs, but in most breeds these are more prominent in males. A muff or beard is a mutation found in several chicken breeds which causes extra feathering under the chicken's face, giving the appearance of a beard.
Domestic chickens are not capable of long distance flight, although lighter birds are capable of flying for short distances, such as over fences or into trees. Chickens may fly to explore their surroundings, but do so only to flee perceived danger. Chickens live together in flocks, they have a communal approach to the incubation of eggs and raising of young. Individual chickens in a flock will dominate others, establishing a "pecking order", with dominant individuals having priority for food access and nesting locations. Removing hens or roosters from a flock causes a temporary disruption to this social order until a new pecking order is established. Adding hens younger birds, to an existing flock can lead to fighting and injury; when a rooster finds food, he may call other chickens to eat first. He does this by clucking in a high pitch as well as dropping the food; this behaviour may be observed in mother hens to call their chicks and encourage them to eat. A rooster's crowing is a loud and sometimes shrill call and sends a territorial signal to other roosters.
However, roosters may crow in response to sudden disturbances within their surroundings. Hens cluck loudly after laying an egg, to call their chicks. Chickens give different warning calls when they sense a predator approaching from the air or on the ground. To initiate courting, some roosters may dance in a circle around or near a hen lowering the wing, closest to the hen; the dance triggers a response in the hen and when she responds to his "call", the rooster may mount the hen and proceed with the mating. More matin
The cerebellar vermis is located in the medial, cortico-nuclear zone of the cerebellum, which resides in the posterior fossa of the cranium. The primary fissure in the vermis curves ventrolaterally to the superior surface of the cerebellum, dividing it into anterior and posterior lobes. Functionally, the vermis is associated with bodily locomotion; the vermis is included within the spinocerebellum and receives somatic sensory input from the head and proximal body parts via ascending spinal pathways. The cerebellum develops in a rostro-caudal manner, with rostral regions in the midline giving rise to the vermis, caudal regions developing into the cerebellar hemispheres. By 4 months of prenatal development, the vermis becomes foliated, while development of the hemispheres lags by 30–60 days. Postnatally and organization of the cellular components of the cerebellum continues, with completion of the foliation pattern by 7 months of life and final migration and arborization of cerebellar neurons by 20 months.
Inspection of the posterior fossa is a common feature of prenatal ultrasound and is used to determine whether excess fluid or malformations of the cerebellum exist. Anomalies of the cerebellar vermis are diagnosed in this manner and include phenotypes consistent with Dandy-Walker malformation, rhombencephalosynapsis, displaying no vermis with fusion of the cerebellar hemispheres, pontocerebellar hypoplasia, or stunted growth of the cerebellum, neoplasms. In neonates, hypoxic injury to the cerebellum is common, resulting in neuronal loss and gliosis. Symptoms of these disorders range from mild loss of fine motor control to severe mental retardation and death. Karyotyping has shown that most pathologies associated with the vermis are inherited though an autosomal recessive pattern, with most known mutations occurring on the X chromosome; the vermis is intimately associated with all regions of the cerebellar cortex, which can be divided into three functional parts, each having distinct connections with the brain and spinal cord.
These regions are the vestibulocerebellum, responsible for the control of eye movements. The vermis is the median portion of the cerebellum that connects the two hemispheres. Both the vermis and the hemispheres are composed of lobules formed by groups of folia. There are nine lobules of the vermis: lingula, central lobule, clivus, folium of the vermis, pyramid and nodule; these lobules are difficult to observe during human anatomy classes and may vary in size and number of folia. It has been shown that folia of the cerebellum exhibit frequent variations in form and arrangement between individuals; the lingula is the first lobule of the upper portion of the vermis on the superoinferior axis and pertains to the paleocerebellum together with the central lobule, culmen and uvula. It is separated from the central lobule by the pre-central fissure; the central lobule is the second lobule of the upper portion of the vermis on the superoinferior axis. The culmen is the third and largest lobule of the upper portion of the vermis on the superoinferior axis.
It is separated from the declive by the primary fissure and is related with the anterior quadrangular lobule of the hemisphere. The pyramid is the seventh lobule of the vermis on the superoinferior axis, it is separated from the uvula by the pre-pyramidal and secondary fissures, respectively. This lobule is related with the biventral lobule of the hemisphere; the uvula is the second largest lobule, following the culmen. It is separated from the nodule by the posterolateral fissure; the spinocerebellum receives proprioception input from the dorsal columns of the spinal cord and from the trigeminal nerve, as well as from visual and auditory systems. It sends fibers to deep cerebellar nuclei that, in turn, project to both the cerebral cortex and the brain stem, thus providing modulation of descending motor systems; this region comprises the intermediate parts of the cerebellar hemispheres. Sensory information from the periphery and from the primary motor and somatosensory cortex terminate in this region.
Purkinje cells of the vermis project to the fastigial nucleus, controlling the axial and proximal musculature involved in the execution of limb movements. Purkinje cells in the intermediate zone of the spinocerebellum project to the interposed nuclei, which control the distal musculature components of the descending motor pathways needed for limb movement. Both of these nuclei include projections to the motor cortex in the cerebrum; the interposed nucleus is smaller than the dentate nucleus but larger than the fastigial nucleus and functions to modulate muscle stretch reflexes of distal musculature. It is located dorsal to lateral to the fastigial nucleus; the fastigial nucleus is the most medial efferent cerebellar nucleus, targeting the pontine and medullary reticular formation as well as the vestibular nuclei. This region deals with antigravity muscle groups and other synergies involved with standing and walking, it is thought that fastigial nuclei axons are excitatory and project beyond the cerebellum using glutamate and aspartate as neurotransmitters.
Malformations of the posterior fossa have been recognized more during the past few decades as the result of recent advances in technology. Malformations of
Anatomical terms of location
Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. All vertebrates have the same basic body plan – they are bilaterally symmetrical in early embryonic stages and bilaterally symmetrical in adulthood; that is, they have mirror-image left and right halves if divided down the middle. For these reasons, the basic directional terms can be considered to be those used in vertebrates. By extension, the same terms are used for many other organisms as well. While these terms are standardized within specific fields of biology, there are unavoidable, sometimes dramatic, differences between some disciplines. For example, differences in terminology remain a problem that, to some extent, still separates the terminology of human anatomy from that used in the study of various other zoological categories. Standardized anatomical and zoological terms of location have been developed based on Latin and Greek words, to enable all biological and medical scientists to delineate and communicate information about animal bodies and their component organs though the meaning of some of the terms is context-sensitive.
The vertebrates and Craniata share a substantial heritage and common structure, so many of the same terms are used for location. To avoid ambiguities this terminology is based on the anatomy of each animal in a standard way. For humans, one type of vertebrate, anatomical terms may differ from other forms of vertebrates. For one reason, this is because humans have a different neuraxis and, unlike animals that rest on four limbs, humans are considered when describing anatomy as being in the standard anatomical position, thus what is on "top" of a human is the head, whereas the "top" of a dog may be its back, the "top" of a flounder could refer to either its left or its right side. For invertebrates, standard application of locational terminology becomes difficult or debatable at best when the differences in morphology are so radical that common concepts are not homologous and do not refer to common concepts. For example, many species are not bilaterally symmetrical. In these species, terminology depends on their type of symmetry.
Because animals can change orientation with respect to their environment, because appendages like limbs and tentacles can change position with respect to the main body, positional descriptive terms need to refer to the animal as in its standard anatomical position. All descriptions are with respect to the organism in its standard anatomical position when the organism in question has appendages in another position; this helps avoid confusion in terminology. In humans, this refers to the body in a standing position with arms at the side and palms facing forward. While the universal vertebrate terminology used in veterinary medicine would work in human medicine, the human terms are thought to be too well established to be worth changing. Many anatomical terms can be combined, either to indicate a position in two axes or to indicate the direction of a movement relative to the body. For example, "anterolateral" indicates a position, both anterior and lateral to the body axis. In radiology, an X-ray image may be said to be "anteroposterior", indicating that the beam of X-rays pass from their source to patient's anterior body wall through the body to exit through posterior body wall.
There is no definite limit to the contexts in which terms may be modified to qualify each other in such combinations. The modifier term is truncated and an "o" or an "i" is added in prefixing it to the qualified term. For example, a view of an animal from an aspect at once dorsal and lateral might be called a "dorsolateral" view. Again, in describing the morphology of an organ or habitus of an animal such as many of the Platyhelminthes, one might speak of it as "dorsiventrally" flattened as opposed to bilaterally flattened animals such as ocean sunfish. Where desirable three or more terms may be agglutinated or concatenated, as in "anteriodorsolateral"; such terms sometimes used to be hyphenated. There is however little basis for any strict rule to interfere with choice of convenience in such usage. Three basic reference planes are used to describe location; the sagittal plane is a plane parallel to the sagittal suture. All other sagittal planes are parallel to it, it is known as a "longitudinal plane".
The plane is perpendicular to the ground. The median plane or midsagittal plane is in the midline of the body, divides the body into left and right portions; this passes through the head, spinal cord, and, in many animals, the tail. The term "median plane" can refer to the midsagittal plane of other structures, such as a digit; the frontal plane or coronal plane divides the body into ventral portions. For post-embryonic humans a coronal plane is vertical and a transverse plane is horizontal, but for embryos and quadrupeds a coronal plane is horizontal and a transverse plane is vertical. A longitudinal plane is any plane perpendicular to the transverse plane; the coronal plane and the sagittal plane are examples of longitudinal planes. A transverse plane known as a cross-section, divides the body into cranial and caudal portions. In human anatomy: A transverse plane is an X-Z plane, parallel to the ground, which s