出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2017/08/21 10:12:32」(JST)
鉤爪（かぎづめ）は、根元から先にかけて割と強く湾曲した爪のこと。正確には哺乳類の爪の構造の一つであるが、それと同じような構造の脊椎動物の爪をいうことが多い。英語で「claw」と呼ばれる類の爪である。爬虫類や鳥類などの爪はどれも鉤爪であり、これが本来の爪の形であったと考えられる。蹄や扁爪は哺乳類において特に進化したものである。
昆虫の肢の先端の器官も鉤爪と呼ぶ場合がある。
扁爪より少し厚く、指趾骨の前半部を覆っている。爪の表面を覆う爪板は前後左右に弓なりに曲がっており、特に先端は下に曲がって鋭く尖っている。また、他の種族の爪より幅が著しく狭い。鉤爪の基部の下方部は皮膚の膨らみがある。これは肉球の一種で指球と呼ばれるものである。歩くときにここが地面に付く様に歩行する。
一般にこれを地面に引っかけて歩行の助けとする。樹上性のものはこれを樹皮などに引っかけることで体を支える。時には天敵などに傷を負わせるなどの役割を持つ。
捕食性のものでは、獲物を捕えるための装置として用いる。特にネコ科の動物は爪を捕獲によく用いるが、普段は爪が指先に引っ込むようになっている。これは歩行中に地面に触れることで爪先の鋭さが鈍らないための適応だと考えられる。また、爪とぎをして手入れする。イヌ科の動物の場合、爪は常に露出しており、走るときにはスパイクのような役割をすると考えられる。しかし、前足の親指だけはやや上に位置しているので爪先は鋭く、獲物を抱え込むときにはこれをよく使う。
小型の肉食恐竜にも、後肢の特定の爪だけが鉤爪のようによく発達したものがあり、やはり捕獲用に用いられたと思われる。これらは歩行時にはこの指を立て、先端が地面に触れないようにしていたのではないかと考えられている。
爬虫類がすべて鉤爪を持つことから見ても、爪の本来の形がこれであると考えられる。哺乳類においても、有袋類や食虫類など、原始的と考えられる群は鉤爪である。したがって、扁爪や蹄を持つものはこのようなものから進化してきたものと考えられる。中には、扁爪や蹄を持ちながら一部に鉤爪を持つものがある。たとえばサル目は扁爪を持つが、キツネザルやロリスなどの原猿類は、いずれもどれか一本の指に鉤爪を残しており、原始的特徴の一つと考えられている。
昆虫などの無脊椎動物において、肢の先端にある鉤状の小突起のことを指す場合がある。体表のクチクラが特に肥厚・伸長したもので、物を引っ掛けるのに便利で移動・攻撃・防御などに用いる。一般的には1本あるいは1対構造となっているが、蜘蛛類のように3本あって糸を編むのに用いる例外もある。
動物の鉤爪を模した道具もある。詳しくは「鉤」参照。
A claw is a curved, pointed appendage, found at the end of a toe or finger in most amniotes (mammals, reptiles, birds).
Some invertebrates such as beetles and spiders have somewhat similar fine hooked structures at the end of the leg or tarsus for gripping a surface as the creature walks. Crabs', lobsters' and scorpions' pincers, or more formally, their chelae, are sometimes called claws.
A true claw is made of hard protein called keratin. Claws are used to catch and hold prey in carnivorous mammals such as cats and dogs, but may also be used for such purposes as digging, climbing trees, self-defense, and grooming, in those and other species.
Similar appendages that are flat and do not come to a sharp point are called nails instead. Claw-like projections that do not form at the end of digits, but spring from other parts of the foot are properly named spurs.[1]
In tetrapods, claws are made of keratin and consist of two layers. The unguis is the harder external layer, which consists of keratin fibers arranged perpendicular to the direction of growth and in layers at an oblique angle. The subunguis is the softer, flaky underside layer whose grain is parallel to the direction of growth. The claw grows outward from the nail matrix at the base of the unguis and the subunguis grows thicker while travelling across the nail 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.
All Carnivora have claws, which vary considerably in length and shape. Claws grow out of the third phalanges of the paws and are made of keratin. Many predatory mammals have protractile claws that can partially hide inside the animal's paw, especially the cat family, Felidae, almost all of which members have fully protractible claws. Outside of the cat family, retractable claws are found only in certain species of the Viverridae (and the extinct Nimravidae).[2] A claw that is retractable is protected from wear and tear.
Most cats and dogs also have a dewclaw on 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.
A nail is homologous to 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 of artiodactyl ungulates may also be called a claw).
Every so often, the growth of claws stops and restarts, as does hair. In hair, this results in the hair falling out and being replaced by a new one. In claws, this results in an abscission layer, and the old segment breaks off. This process takes several months for human thumbnails. Cats are 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. Domesticated equids (horses, donkeys and mules) usually need regular trimming by a farrier, as a consequence of reduced activity on hard ground.
Primate nails 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 exhibit nails. However, claw-like nails are found in small-bodied callitrichids on all digits except the hallux or big toe. A laterally flattened grooming claw, used for grooming, can be found on the second toe in living strepsirrhines, and the second and third in tarsiers. Aye-ayes have functional claws on all other digits except the hallux, including a grooming claw on the second toe.[3] Less commonly known, a grooming claw is also found on the second pedal digit of night monkeys (Aotus), titis (Callicebus), and possibly other New World monkeys.[4]
Most lizards have toes ending in stout claws. The claws form from the last scale on the toe.[5] Most reptiles have well-developed claws. In snakes, feet and claws are absent, but in many boids such as Boa 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.
A talon is the claw of a bird of prey, its primary hunting tool.[6] 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. Cassowaries use claws on their inner toe (digit II) 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.
The hoatzin and turaco are unique among extant birds in having functional claws on the thumb and index finger (digit I and II) on the forelimbs as chicks, allowing them to climb trees until the adult plumage with flight feathers develop.[7][8] 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.[9]
The only amphibians to bear claws are the African clawed frogs. Claws appear to have evolved separately in the amphibian and amniote line.[10]
The correct term for the "claw" of an arthropod, such as a lobster or crab, is a chela (plural chelae). Legs bearing a chela are called chelipeds. Chelae are also called pincers.
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| リンク元 | 「hoof」「hooves」 |