Claw

Aclawis a curved, pointed appendage found at the end of a toe or finger in mostamniotes(mammals,reptiles,birds). Someinvertebratessuch asbeetlesandspidershave somewhat similar fine, hooked structures at lower elevations that allow the spider cells of their own species the end of the leg ortarsusfor gripping a surface as they walk. The pincers ofcrabs,lobstersandscorpions, more formally known as theirchelae, are sometimes called claws.
A true claw is made of a hardproteincalledkeratin. Claws are used to catch and hold prey incarnivorousmammalssuch ascatsanddogs, but may also be used for such purposes as digging, climbing trees, self-defense andgrooming, in those and other species.
Similar appendages that are flat and do not come to a sharp point are callednailsinstead. Claw-like projections that do not form at the end of digits but spring from other parts of the foot are properly namedspurs.
Intetrapods, claws are made ofkeratinand consist of two layers. Theunguisis the harder external layer, which consists of keratin fibers arranged perpendicular to the direction of growth and in layers at an oblique angle. Thesubunguisis the softer, flaky underside layer whose grain is parallel to the direction of growth. The claw grows outward from thenail matrixat the base of the unguis and the subunguis grows thicker while travelling across thenail bed. The unguis grows outward faster than the subunguis to produce a curve and the thinner sides of the claw wear away faster than their thicker middle, producing a more or less sharp point. Tetrapods use their claws in many ways, commonly to grasp or kill prey, to dig and to climb and hang.
Allcarnivoranshave claws, which vary considerably in length and shape. Claws grow out of the thirdphalangesof the paws and are made ofkeratin. Manypredatorymammals haveprotractileclaws that can partially hide inside the animal'spaw, especially the cat family,Felidae, almost all of whose members have fully protractible claws. Outside of the cat family, retractable claws are found only in certain species of theViverridae(and the extinctNimravidae).A claw that is retractable is protected from wear and tear.
Most cats and dogs also have adewclawon the inside of the front paws. It is much less functional than the other claws but does help the cats to grasp prey. Because the dew claw does not touch the ground, it receives less wear and tends to be sharper and longer.
Anailishomologousto a claw but is flatter and has a curved edge instead of a point. A nail that is big enough to bear weight is called a "hoof". (Nevertheless, one side of the cloven-hoof ofartiodactylungulatesmay also be called a claw).
Every so often, the growth of claws stops and restarts, as doeshair. In a hair, this results in the hair falling out and being replaced by a new one. In claws, this results in anabscissionlayer, and the old segment breaks off. This process takes several months forhumanthumbnails.Catsare often seen working old unguis layers off on wood or on boards made for the purpose. Ungulates' hooves wear or self-trim by ground contact. Domesticatedequids(horses,donkeysandmules) usually need regular trimming by afarrier, as a consequence of reduced activity on hard ground.
Primatenails consist of the unguis alone, as the subunguis has disappeared. With the evolution of grasping hands and feet, claws are no longer necessary for locomotion, and instead most digits exhibitnails. However, claw-like nails are found in small-bodiedcallitrichidson all digits except thehalluxor big toe. A laterally flattenedgrooming claw, used for grooming, can be found on the second toe in livingstrepsirrhines, and the second and third intarsiers.Aye-ayeshave functional claws on all other digits except the hallux, including a grooming claw on the second toe.Less commonly known, a grooming claw is also found on the second pedal digit ofnight monkeys(Aotus),titis(Callicebus), and possibly otherNew World monkeys.
Most reptiles have well-developed claws. Most lizards have toes ending in stout claws.In snakes, feet and claws are absent, but in manyboidssuch asBoa constrictor, remnants of highly reduced hind-limbs emerge with a single claw as "spurs" on each side of the anal opening.
Lizard claws are used as aids in climbing, and in holding down prey in carnivorous species.
Atalonis the claw of abird of prey, its primary hunting tool.The talons are very important; without them, most birds of prey would not be able to catch their food. Some birds also use claws for defensive purposes.Cassowariesuse claws on their inner toe (digit I) for defence and have been known to disembowel people.All birds, however, have claws, which are used as general holdfasts and protection for the tip of the digits.
Thehoatzinandturacoare unique amongextantbirds in having functional claws on the thumb and index finger (digits I and II) on the forelimbs as chicks, allowing them to climb trees until the adult plumage with flight feathers develop.However, several birds have a claw- or nail-like structure hidden under the feathers at the end of the hand digits, notably ostriches, emus, ducks, geese and kiwis.
The onlyamphibiansto bear claws are theAfrican clawed frogs. Claws evolved separately in the amphibian andamniote(reptiliomorph) line.However, thehairy froghas claw analogues on its feet; the frog intentionallydislocatesthe tips of its fingers to unsheathe the sharp points of its lastphalanges.
The scientifically correct term for the "claw" of an arthropod, such as a lobster or crab, is achela(pluralchelae). Legs bearing a chela are calledchelipeds. Chelae are also calledpincers.