rabbits are called aradicular hypsodont teeth. this is sometimes referred to as an elodent dentition. these teeth grow or erupt continuously. the growth or eruption is held in balance by dental abrasion from chewing a diet high in fiber.buccal view of top incisor from rattus rattus. top incisor outlined in yellow. molars circled in blue.buccal view of the lower incisor from the right dentary of a rattus rattus lingual view of the lower incisor from the right dentary of a rattus rattus midsagittal view of top incisor from rattus rattus. top incisor outlined in yellow. molars circled in blue. rodents rodents have upper and lower hypselodont incisors that can continuously grow enamel throughout its life without having properly formed roots. these teeth are also known as aradicular teeth, and unlike humans whose ameloblasts die after tooth development, rodents continually produce enamel, they must wear down their teeth by gnawing on various materials. enamel and dentin are produced by the enamel organ, and growth is dependent on the presence of stem cells, cellular amplification, and cellular maturation structures in the odontogenic region. rodent incisors are used for cutting wood, biting through the skin of fruit, or for defense. this allows for the rate of wear and tooth growth to be at equilibrium. the microstructure of rodent incisor enamel has shown to be useful in studying the phylogeny and systematics of rodents because of its independent evolution from the other dental traits. the enamel on rodent incisors are composed of two layers: the inner portio interna (pi) with hunter-schreger bands (hsb) and an outer portio externa (pe) with radial enamel (re). it usually involves the differential regulation of the epithelial stem cell niche in the tooth of two rodent species, such as guinea pigs. lingual view of top incisor from rattus rattus. top incisor outlined in yellow. molars circled in blue. the teeth have enamel on the outside and exposed dentin on the inside, so they self-sharpen during gnawing. on the other hand, continually growing molars are found in some rodent species, such as the sibling vole and the guinea pig. there is variation in the dentition of the rodents, but generally, rodents lack canines and premolars, and have a space between their incisors and molars, called the diastema region. manatee manatees are polyphyodont with mandibular molars developing separately from the jaw and are encased in a bony shell separated by soft tissue. walrus walrus tusks are canine teeth that grow continuously throughout life. fish teeth of great white shark fish, such as sharks, may go through many teeth in their lifetime. the replacement of multiple teeth is known as polyphyodontia. a class of prehistoric shark are called cladodonts for their strange forked teeth. unlike the continuous shedding of functional teeth seen in modern sharks, the majority of stem chondrichthyan lineages retained all tooth generations developed throughout the life of the animal. this replacement mechanism is exemplified by the tooth whorl-based dentitions of acanthodians, which include the oldest known toothed vertebrate, qianodus duplicis. amphibians all amphibians have pedicellate teeth which are modified to be flexible due to connective tissue and uncalcified dentine that separates the crown from the base of the tooth. most amphibians exhibit teeth that have a slight attachment to the jaw or acrodont teeth. acrodont teeth exhibit limited connection to the dentary and have little enervation. this is ideal for organisms who mostly use their teeth for grasping, but not for crushing and allows for rapid regeneration of teeth at a low energy cost. teeth are usually lost in the course of feeding if the prey is struggling. additionally, amphibians that undergo a metamorphosis develop bicuspid shaped teeth. reptiles the teeth of reptiles are replaced constantly throughout their lives. crocodilian juveniles replace teeth with larger ones at a rate as high as one new tooth per socket every month. once mature, tooth replacement rates can slow to two years and even longer. overall, crocodilians may use 3,000 teeth from birth to death. new teeth are created within old teeth. birds a skull of ichthyornis discovered in 2014 suggests that the beak of birds may have evolved from teeth to allow chicks to escape their shells earlier, and thus avoid predators and also to penetrate protective covers such as hard earth to access underlying food. invertebrates the european medicinal leech has three jaws with numerous sharp teeth which function like little saws for incising a host. true teeth are unique to vertebrates, although many invertebrates have analogous structures often referred to as teeth. the organisms with the simplest genome bearing such tooth-like structures are perhaps the parasitic worms of the family ancylostomatidae. for example, the hookworm necator americanus has two dorsal and two ventral cutting plates or teeth around the anterior margin of the buccal capsule. it also has a pair of subdorsal and a pair of subventral teeth located close to the rear. historically the european medicinal leech, another invertebrate parasite, has been used in medicine to remove blood from patients. they have three jaws (tripartite) that resemble saws in both appearance and function, and on them are about 100 sharp teeth used to incise the host. the incision leaves a mark that is an inverted y inside of a circle. after piercing the skin and injecting anticoagulants (hirudin) and anaesthetics, they suck out blood, consuming up to ten times their body weight in a single meal. in some species of bryozoa, the first part of the stomach forms a muscular gizzard lined with chitinous teeth that crush armoured prey such as diatoms. wave-like peristaltic contractions then move the food through the stomach for digestion. the limpet rasps algae from rocks using teeth with the strongest known tensile strength of any biological material. molluscs have a structure called a radula which bears a ribbon of chitinous teeth. however, these teeth are histologically and developmentally different from vertebrate teeth and are unlikely to be homologous. for example, vertebrate teeth develop from a neural crest mesenchyme-derived dental papilla, and the neural crest is specific to vertebrates, as are tissues such as enamel. the radula is used by molluscs for feeding and is sometimes compared rather inaccurately to a tongue. it is a minutely toothed, chitinous ribbon, typically used for scraping or cutting food before the food enters the oesophagus. the radula is unique to molluscs, and is found in every class of mollusc apart from bivalves. within the gastropods, the radula is used in feeding by both herbivorous and carnivorous snails and slugs. the arrangement of teeth (also known as denticles) on the radula ribbon varies considerably from one group to another as shown in the diagram on the left. predatory marine snails such as the naticidae use the radula plus an acidic secretion to bore through the shell of other molluscs. other predatory marine snails, such as the conidae, use a specialized radula tooth as a poisoned harpoon. predatory pulmonate land slugs, such as the ghost slug, use elongated razor-sharp teeth on the radula to seize and devour earthworms. predatory cephalopods, such as squid, use the radula for cutting prey. in most of the more ancient lineages of gastropods, the radula is used to graze by scraping diatoms and other microscopic algae off rock surfaces and other substrates. limpets scrape algae from rocks using radula equipped with exceptionally hard rasping teeth. these teeth have the strongest known tensile strength of any biological material, outperforming spider silk. the mineral protein of the limpet teeth can withstand a tensile stress of 4.9 gpa, compared to 4 gpa of spider silk and 0.5 gpa of human teeth. fossilization and taphonomy because teeth are very resistant, often preserved when bones are not, and reflect the diet of the host organism, they are very valuable to archaeologists and palaeontologists. early fish such as the thelodonts had scales composed of dentine and an enamel-like compound, suggesting that the origin of teeth was from scales which were retained in the mouth. fish as early as the late cambrian had dentine in their exoskeletons, which may have functioned in defense or for sensing their environments. dentine can be as hard as the rest of teeth and is composed of collagen fibres, reinforced with hydroxyapatite. though teeth are very resistant, they also can be brittle and highly susceptible to cracking. however, cracking of the tooth can be used as a diagnostic tool for predicting bite force. additionally, enamel fractures can also give valuable insight into the diet and behaviour of archaeological and fossil samples. decalcification removes the enamel from teeth and leaves only the organic interior intact, which comprises dentine and cementine. enamel is quickly decalcified in acids, perhaps by dissolution by plant acids or via diagenetic solutions, or in the stomachs of vertebrate predators. enamel can be lost by abrasion or spalling, and is lost before dentine or bone are destroyed by the fossilisation process. in such a case, the 'skeleton' of the teeth would consist of the dentine, with a hollow pulp cavity. the organic part of dentine, conversely, is destroyed by alkalis.