Countershading, or Thayer's law, is a method of camouflage in which an animal's coloration is darker on the upper side and lighter on the underside of the body. This pattern is found in many species of mammals, birds and insects, both predators and prey, has occurred since at least the Cretaceous period; when light falls from above on a uniformly coloured three-dimensional object such as a sphere, it makes the upper side appear lighter and the underside darker, grading from one to the other. This pattern of light and shade makes the object appear solid, therefore easier to detect; the classical form of countershading, discovered in 1909 by the artist Abbott Handerson Thayer, works by counterbalancing the effects of self-shadowing, again with grading from dark to light. In theory this could be useful for military camouflage, but in practice it has been applied, despite the best efforts of Thayer and in the Second World War, of the zoologist Hugh Cott; the precise function of various patterns of animal coloration that have been called countershading has been debated by zoologists such as Hannah Rowland, with the suggestion that there may be multiple functions including flattening and background matching when viewed from the side.
A related mechanism, counter-illumination, adds the creation of light by bioluminescence or lamps to match the actual brightness of a background. Counter-illumination camouflage is common in marine organisms such as squid, it has been studied up to the prototype stage for military use in ships and aircraft, but it too has or never been used in warfare. The reverse of countershading, with the belly pigmented darker than the back, enhances contrast and so makes animals more conspicuous, it is found in animals, such as skunks. The pattern is used both in startle or deimatic displays and as a signal to warn off experienced predators. However, animals that habitually live upside-down but lack strong defences, such as the Nile catfish and the Luna moth caterpillar, have upside-down countershading for camouflage; the English zoologist Edward Bagnall Poulton, author of The Colours of Animals discovered the countershading of various insects, including the pupa or chrysalis of the purple emperor butterfly, Apatura iris, the caterpillar larvae of the brimstone moth, Opisthograptis luteolata and of the peppered moth, Biston betularia.
However he did he suggest that the effect occurred widely. The New Hampshire artist Abbott Handerson Thayer was one of the first to study and write about countershading. In his 1909 book Concealing-Coloration in the Animal Kingdom, he described and illustrated countershading with photographs and paintings, but wrongly claimed that all animals are countershaded. For this reason countershading is sometimes called Thayer's law. Thayer wrote: Animals are painted by Nature darkest on those parts which tend to be most lighted by the sky's light, vice versa.... The fact that a vast majority of creatures of the whole animal kingdom wear this gradation, developed to an exquisitely minute degree, are famous for being hard to see in their homes, speaks for itself. Thayer observed and painted a number of examples, including the Luna moth caterpillar Actias luna, both in its habitual upside-down feeding position, where its countershading makes it appear flat, artificially inverted from that position, where sunlight and its inverted countershading combine to make it appear shaded and therefore solid.
Thayer obtained a patent in 1902 to paint warships, both submarines and surface ships, using countershading, but failed to convince the US Navy to adopt his ideas. Hugh Bamford Cott in his 1940 book Adaptive Coloration in Animals described many instances of countershading, following Thayer in general approach but criticising Thayer's excessive claim that all animals are camouflaged with countershading. Cott called this "Thayer straining the theory to a fantastic extreme". Both Thayer and Cott included in their books photographs of a non-countershaded white cockerel against a white background, to make the point that in Thayer's words "a monochrome object can not be'obliterated', no matter what its background" or in Cott's words "Colour resemblance alone is not sufficient to afford concealment". Cott explained that Contrary to what might have been expected by any one lacking in artistic perception, the bird appears conspicuous, the back looking lighter, the breast darker, than the background, although in actual fact, back and breast are all pure white."
Countershading is observed in a wide range of animal groups, both terrestrial, such as deer, marine, such as sharks. It prey, it is used alongside other forms of camouflage including colour matching and disruptive coloration. Among predatory fish, the gray snapper, Lutianus griseus, is flattened by its countershading, while it hunts an "almost invisible" prey, the hardhead fish, Atherina laticeps which swims over greyish sands. Other countershaded marine animals include blue shark and dolphin, it tones the canvas on which are painted the Leopard's spots, the Tiger's stripes... It is the dress universally worn by rodents... It is the essential uniform adopt
The jaw is any opposable articulated structure at the entrance of the mouth used for grasping and manipulating food. The term jaws is broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of most animals. In arthropods, the jaws are chitinous and oppose laterally, may consist of mandibles or chelicerae; these jaws are composed of numerous mouthparts. Their function is fundamentally for food acquisition, conveyance to the mouth, and/or initial processing. Many mouthparts and associate structures are modified legs. In most vertebrates, the jaws are bony or cartilaginous and oppose vertically, comprising an upper jaw and a lower jaw; the vertebrate jaw is derived from the most anterior two pharyngeal arches supporting the gills, bears numerous teeth. The vertebrate jaw originally evolved in the Silurian period and appeared in the Placoderm fish which further diversified in the Devonian; the two most anterior pharyngeal arches are thought to have become the jaw itself and the hyoid arch, respectively.
The hyoid system suspends the jaw from the braincase of the skull, permitting great mobility of the jaws. While there is no fossil evidence directly to support this theory, it makes sense in light of the numbers of pharyngeal arches that are visible in extant jawed vertebrates, which have seven arches, primitive jawless vertebrates, which have nine; the original selective advantage offered by the jaw may not be related to feeding, but rather to increased respiration efficiency. The jaws were used in the buccal pump that pumps water across the gills of fish or air into the lungs in the case of amphibians. Over evolutionary time the more familiar use of jaws, in feeding, was selected for and became a important function in vertebrates. Many teleost fish have modified jaws for suction feeding and jaw protrusion, resulting in complex jaws with dozens of bones involved; the jaw in tetrapods is simplified compared to fish. Most of the upper jaw bones have been fused to the braincase, while the lower jaw bones have been fused together into a unit called the mandible.
The jaw articulates via a hinge joint between the articular. The jaws of tetrapods exhibit varying degrees of mobility between jaw bones; some species have jaw bones fused, while others may have joints allowing for mobility of the dentary, quadrate, or maxilla. The snake skull shows the greatest degree of cranial kinesis, which allows the snake to swallow large prey items. In mammals the jaws are made up of the maxilla. In the ape there is a reinforcement to the lower jaw bone called the simian shelf. In the evolution of the mammalian jaw, two of the bones of the jaw structure were reduced in size and incorporated into the ear, while many others have been fused together; as a result, mammals show little or no cranial kinesis, the mandible is attached to the temporal bone by the temporomandibular joints. Temporomandibular joint dysfunction is a common disorder of these joints, characterized by pain and limitation of mandibular movement. Sea urchins possess unique jaws which display five-part symmetry, termed the Aristotle's lantern.
Each unit of the jaw holds a single, perpetually growing tooth composed of crystalline calcium carbonate. Muscles of mastication Otofacial syndrome Predentary Prognathism Rostral bone Jaw at the US National Library of Medicine Medical Subject Headings
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
A pika is a small mammal, with short limbs round body, rounded ears, no external tail. They with shorter ears, they live in mountainous countries in Asia, there are two species in North America. Most pikas prefer rocky slopes; the large-eared pika of the Himalayas and nearby mountains is one of the highest living mammals. Pikas graze on a range of plants grasses and young stems. In the autumn, they pull hay, soft twigs and other stores of food into their burrows to eat during the long, cold winter; the name "pika" is used for any member of a family within the order of lagomorphs. One genus, Ochotona, is recognised within the family, it includes 30 species, it is known as the "whistling hare" due to its high-pitched alarm call when diving into its burrow. In the United States, the pika is colloquially called a "coney", a nonspecific term used for rabbits and hyraxes; the name "pika" appears to be derived from the Tungus piika and the scientific name Ochotona is from the Mongolian word ogdoi which means pika.
Pikas are native to cold climates in Asia, North America, parts of Eastern Europe. Most species live on rocky mountainsides, where there are numerous crevices in which to shelter, although some pikas construct crude burrows. A few burrowing species are native to open steppe land. In the mountains of Eurasia, pikas share their burrows with snowfinches, which build their nests there. Pikas require cold temperatures to live, can die if exposed to temperatures above 77.9 °F. Changing temperatures have forced some pika populations to restrict their ranges to higher elevations. Pikas rounded ears, they are about 15 to 23 centimetres in body length and weigh between 120 and 350 grams, depending on species. Like rabbits, after eating they produce soft green feces, which they eat again to take in further nutrition, before producing the final, fecal pellets; some pikas, such as the collared pika, have been known to store dead birds in their burrows for food during winter. These animals are herbivores, feed on a wide variety of plant matter, including forbs, sedges, shrub twigs and lichen.
As with other lagomorphs, pikas have gnawing incisors and no canines, although they have fewer molars than rabbits, have a dental formula of: 188.8.131.52.0.2.3 Rock-dwelling pikas have small litters of fewer than five young, while the burrowing species tend to give birth to more young, to breed more possibly due to a greater availability of resources in their native habitats. The young are born after a gestation period of between 30 days. Pikas are diurnal or crepuscular, with higher-elevation species being more active during the daytime, they show their peak activity just before the winter season. Pikas do not hibernate, so they spend time during the summer collecting and storing food they will eat over the winter; each rock-dwelling pika stores its own "haypile" of dried vegetation, while burrowing species share food stores with their burrow mates. Haying behavior is more prominent at higher elevations. Many of the vocalizations and social behaviors that pikas exhibit are related to haypile defense.
Eurasian pikas live in family groups and share duties of gathering food and keeping watch. Some species are territorial. North American pikas are asocial. Pikas have distinct calls; the call can either be short and quick, a little longer and more drawn out. The short calls are an example of geographic variation; the pikas determine the appropriate time to make short calls by listening for cues for sound localization. The calls are used for individual recognition, predator warning signals, territory defense, or as a way to attract the opposite sex. There are different calls depending on the season. In the spring, the songs become more frequent during the breeding season. In late summer, the vocalizations become short calls. Through various studies, the acoustic characteristics of the vocalizations can be a useful taxonomic tool; the average lifespan in pikas is seven years in the wild. A pika's age is determined by the number of adhesion lines on the periosteal bone on the lower jaw; the lifespan does not differ between the sexes.
There are 30 species listed. Order LagomorphaFamily Ochotonidae: pikas Genus Ochotona Subgenus Pika: northern pikas Alpine pika or Altai pika, Ochotona alpina Helan Shan pika or silver pika, Ochotona argentata Collared pika, Ochotona collaris Hoffmann's pika, Ochotona hoffmanni Northern pika or Siberian pika, Ochotona hyperborea Pallas's pika, Ochotona pallasi American pika, Ochotona princeps Turuchan pika, Ochotona turuchanensis Subgenus Ochotona: shrub-steppe pikas Gansu pika or gray pika, Ochotona cansus Plateau pika or black-lipped pika, Ochotona curzoniae Daurian pika, Ochotona dauurica Tsing-ling pika, Ochotona huangensis Nubra pika, Ochotona nubrica Steppe pika, Ochotona pusilla Afghan pika, Ochotona rufescens Moupin pika, Ochotona thibetana Thomas's pika, Ochotona thomasi Ochotona yarlungensis Ochotona qionglaoensis Subgenus Conothoa: mountain pikas Chinese red pika, Ochotona erythrotis Forrest's pika, Ochotona forresti Gaoligong pika, Ochotona gaoligongensis Glover's pika, Ochotona gloveri Himalayan pika, Ochotona himalayana Ili pika, Ochotona iliensis Koslov's pika, Ochotona koslowi Ladak pika, Ochotona ladacensis Large-eared pika, Ochotona macrotis Muli pika, O
Mammals are vertebrate animals constituting the class Mammalia, characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals; the largest orders are the rodents and Soricomorpha. The next three are the Primates, the Cetartiodactyla, the Carnivora. In cladistics, which reflect evolution, mammals are classified as endothermic amniotes, they are the only living Synapsida. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds; the line following the stem group Sphenacodontia split off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals in the early Mesozoic era.
The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, have been among the dominant terrestrial animal groups from 66 million years ago to the present. The basic body type is quadruped, most mammals use their four extremities for terrestrial locomotion. Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale—the largest animal on the planet. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals; the most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation. Most mammals are intelligent, with some possessing large brains, self-awareness, tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals and echolocation.
Mammals can organize themselves into fission-fusion societies and hierarchies—but can be solitary and territorial. Most mammals are polygynous. Domestication of many types of mammals by humans played a major role in the Neolithic revolution, resulted in farming replacing hunting and gathering as the primary source of food for humans; this led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, the development of the first civilizations. Domesticated mammals provided, continue to provide, power for transport and agriculture, as well as food and leather. Mammals are hunted and raced for sport, are used as model organisms in science. Mammals have been depicted in art since Palaeolithic times, appear in literature, film and religion. Decline in numbers and extinction of many mammals is driven by human poaching and habitat destruction deforestation. Mammal classification has been through several iterations since Carl Linnaeus defined the class.
No classification system is universally accepted. George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself through the new concept of cladistics. Though field work made Simpson's classification outdated, it remains the closest thing to an official classification of mammals. Most mammals, including the six most species-rich orders, belong to the placental group; the three largest orders in numbers of species are Rodentia: mice, porcupines, beavers and other gnawing mammals. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes and lemurs. According to Mammal Species of the World, 5,416 species were identified in 2006.
These were grouped into 153 families and 29 orders. In 2008, the International Union for Conservation of Nature completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 extinct; the word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma. In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therian m
The tail is the section at the rear end of certain kinds of animals’ bodies. It is the part of the body that corresponds to the sacrum and coccyx in mammals and birds. While tails are a feature of vertebrates, some invertebrates including scorpions and springtails, as well as snails and slugs, have tail-like appendages that are sometimes referred to as tails. Tailed objects are sometimes referred to as "caudate" and the part of the body associated with or proximal to the tail are given the adjective "caudal". Animal tails are used in a variety of ways, they provide a source of some other forms of marine life. Many land animals use their tails to brush away other biting insects; some species, including cats and kangaroos, use their tails for balance. Tails are used for social signaling; some deer species flash the white underside of their tails to warn other nearby deer of possible danger, beavers slap the water with their tails to indicate danger, canids indicate emotions through the positioning and movement of their tails.
Some species' tails are armored, some, such as those of scorpions, contain venom. Some species of lizard can detach their tails from their bodies; this can help them to escape predators, which are either distracted by the wriggling, detached tail or left with only the tail while the lizard flees. Tails cast in this manner grow back over time, though the replacement is darker in colour than the original. Various species of rat demonstrate a similar function with their tails, known as degloving, in which the outer layer is shed in order for the animal to escape from a predator. Most birds' tails end in long feathers called rectrices; these feathers are used as a rudder, helping the bird maneuver in flight. In some species—such as birds of paradise and most notably peafowl—modified tail feathers play an important role in courtship displays; the extra-stiff tail feathers of other species, including woodpeckers and woodcreepers, allow them to brace themselves against tree trunks. The tails of grazing animals, such as horses, are used both to sweep away insects and positioned or moved in ways that indicate the animal's physical or emotional state.
Human embryos have a tail. As the embryo develops into a fetus, the tail is absorbed by the growing body. Infrequently, a child is born with a ’"soft tail", which contains no vertebrae, but only blood vessels and nerves, but this is regarded as an abnormality rather than a vestigial true tail when such an appendage is located where the tail would be expected. Humans have a "tail bone" attached to the pelvis, formed of fused vertebrae four, at the bottom of the vertebral column, it does not protrude externally. Empennage, the tail of an aircraft Rump Media related to Tails at Wikimedia Commons
A chordate is an animal constituting the phylum Chordata. During some period of their life cycle, chordates possess a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, a post-anal tail: these five anatomical features define this phylum. Chordates are bilaterally symmetric; the Chordata and Ambulacraria together form the superphylum Deuterostomia. Chordates are divided into three subphyla: Vertebrata. There are extinct taxa such as the Vetulicolia. Hemichordata has been presented as a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and Echinodermata form the Ambulacraria, the sister phylum of the Chordates. Of the more than 65,000 living species of chordates, about half are bony fish that are members of the superclass Osteichthyes. Chordate fossils have been found from as early as the Cambrian explosion, 541 million years ago. Cladistically, vertebrates - chordates with the notochord replaced by a vertebral column during development - are considered to be a subgroup of the clade Craniata, which consists of chordates with a skull.
The Craniata and Tunicata compose the clade Olfactores. Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features: A notochord, a stiff rod of cartilage that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the spine, in wholly aquatic species this helps the animal to swim by flexing its tail. A dorsal neural tube. In fish and other vertebrates, this develops into the spinal cord, the main communications trunk of the nervous system. Pharyngeal slits; the pharynx is the part of the throat behind the mouth. In fish, the slits are modified to form gills, but in some other chordates they are part of a filter-feeding system that extracts particles of food from the water in which the animals live. Post-anal tail. A muscular tail that extends backwards behind the anus. An endostyle; this is a groove in the ventral wall of the pharynx. In filter-feeding species it produces mucus to gather food particles, which helps in transporting food to the esophagus.
It stores iodine, may be a precursor of the vertebrate thyroid gland. There are soft constraints that separate chordates from certain other biological lineages, but are not part of the formal definition: All chordates are deuterostomes; this means. All chordates are based on a bilateral body plan. All chordates are coelomates, have a fluid filled body cavity called a coelom with a complete lining called peritoneum derived from mesoderm; the following schema is from the third edition of Vertebrate Palaeontology. The invertebrate chordate classes are from Fishes of the World. While it is structured so as to reflect evolutionary relationships, it retains the traditional ranks used in Linnaean taxonomy. Phylum Chordata †Vetulicolia? Subphylum Cephalochordata – Class Leptocardii Clade Olfactores Subphylum Tunicata – Class Ascidiacea Class Thaliacea Class Appendicularia Class Sorberacea Subphylum Vertebrata Infraphylum incertae sedis Cyclostomata Superclass'Agnatha' paraphyletic Class Myxini Class Petromyzontida or Hyperoartia Class †Conodonta Class †Myllokunmingiida Class †Pteraspidomorphi Class †Thelodonti Class †Anaspida Class †Cephalaspidomorphi Infraphylum Gnathostomata Class †Placodermi Class Chondrichthyes Class †Acanthodii Superclass Osteichthyes Class Actinopterygii Class Sarcopterygii Superclass Tetrapoda Class Amphibia Class Sauropsida Class Synapsida Craniates, one of the three subdivisions of chordates, all have distinct skulls.
They include the hagfish. Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or all deuterostomes, are by their tails". Most craniates are vertebrates; these consist of a series of bony or cartilaginous cylindrical vertebrae with neural arches that protect the spinal cord, with projections that link the vertebrae. However hagfish have incomplete braincases and no vertebrae, are therefore not regarded as vertebrates, but as members of the craniates, the group from which vertebrates are thought to have evolved; however the cladistic exclusion of hagfish from the vertebrates is controversial, as they ma