Qu'est-ce (qui) est placoid - définition
.JPG?width=200 "140px")
![rostral]] end of the skate is to the right.](https://commons.wikimedia.org/wiki/Special:FilePath/Skate Brain Regions.png?width=200 "rostral]] end of the skate is to the right.")
.png?width=200 "140px")
.jpg?width=200 "150px")
![The scales of a typical teleost fish, like this [[Atlantic herring]], are silvered.](https://commons.wikimedia.org/wiki/Special:FilePath/Atlantic herring.jpg?width=200 "The scales of a typical teleost fish, like this [[Atlantic herring]], are silvered.")
.jpg?width=200 "Barnacle growth on boat hull")
![[[Lobe-finned fish]]es, like this preserved [[coelacanth]], have elasmoid scales.}}](https://commons.wikimedia.org/wiki/Special:FilePath/Coelacanth-PaleozoologicalMuseumOfChina-May23-08.jpg?width=200 "[[Lobe-finned fish]]es, like this preserved [[coelacanth]], have elasmoid scales.}}")
 by M. L. Nievera (colored).png?width=200 "url =https://books.google.com/books?id=-4keAAAAIAAJ&q=Eleotris%20butis&pg=PA306}}</ref>")
![Ctenoid scales from a [[perch]] vary from the medial (middle of the fish), to dorsal (top), to caudal (tail end) scales.](https://commons.wikimedia.org/wiki/Special:FilePath/Ctenoid Perch Scales.png?width=200 "Ctenoid scales from a [[perch]] vary from the medial (middle of the fish), to dorsal (top), to caudal (tail end) scales.")
![[[Pineconefish]] are covered in scutes.}}](https://commons.wikimedia.org/wiki/Special:FilePath/FMIB 51542 Pine-cone fish, Monocentris japonicus (Houttnyn) Waka, Japan.jpeg?width=200 "[[Pineconefish]] are covered in scutes.}}")
![Geometrically laid out ganoid scales on a [[bichir]]](https://commons.wikimedia.org/wiki/Special:FilePath/Polypterus bichir from Sudan at Göteborgs Naturhistoriska Museum 9033.jpg?width=200 "Geometrically laid out ganoid scales on a [[bichir]]")
![The scales of this [[spotted gar]] appear glassy due to ganoine.](https://commons.wikimedia.org/wiki/Special:FilePath/Spotted Gar (Lepisosteus oculatus) (3149758934).jpg?width=200 "The scales of this [[spotted gar]] appear glassy due to ganoine.")
![Left to right: denticles of ''[[Paralogania]]'' (?), ''[[Shielia]] taiti'', ''[[Lanarkia]] horrida''](https://commons.wikimedia.org/wiki/Special:FilePath/Thelodont denticles.png?width=200 "Left to right: denticles of ''[[Paralogania]]'' (?), ''[[Shielia]] taiti'', ''[[Lanarkia]] horrida''")
![Cartilaginous fishes, like this [[tiger shark]], have placoid scales (dermal denticles).}}](https://commons.wikimedia.org/wiki/Special:FilePath/Tiger shark.jpg?width=200 "Cartilaginous fishes, like this [[tiger shark]], have placoid scales (dermal denticles).}}")
![The [[alligator gar]] has a tough armouring of [[rhomboidal]]-shaped ganoid scales.](https://commons.wikimedia.org/wiki/Special:FilePath/Alligator gar fish.jpg?width=200 "The [[alligator gar]] has a tough armouring of [[rhomboidal]]-shaped ganoid scales.")
![scute-like]] armour plates.](https://commons.wikimedia.org/wiki/Special:FilePath/Acipenser oxyrhynchus (recropped).png?width=200 "scute-like]] armour plates.")
![The ganoid scales on a [[bowfin]] are reduced in size and resemble [[cycloid scales]].](https://commons.wikimedia.org/wiki/Special:FilePath/Bowfin fish image.jpg?width=200 "The ganoid scales on a [[bowfin]] are reduced in size and resemble [[cycloid scales]].")
![mya]]](https://commons.wikimedia.org/wiki/Special:FilePath/Fish Fossil (FindID 64765).jpg?width=200 "mya]]")
![[[salmon]]}}](https://commons.wikimedia.org/wiki/Special:FilePath/FMIB 49568 -Salmon Scale- 40 lbds August 1906 First return from the sea.jpeg?width=200 "[[salmon]]}}")
![[[bream]]}}](https://commons.wikimedia.org/wiki/Special:FilePath/PSM V35 D072 Scale of bream.jpg?width=200 "[[bream]]}}")
![[[loach]]}}](https://commons.wikimedia.org/wiki/Special:FilePath/PSM V35 D073 Scale of loach.jpg?width=200 "[[loach]]}}")
![[[minnow]]}}](https://commons.wikimedia.org/wiki/Special:FilePath/PSM V35 D073 Scale of minnow.jpg?width=200 "[[minnow]]}}")
![grayling]]}}](https://commons.wikimedia.org/wiki/Special:FilePath/PSM V35 D076 Scale of grayling.jpg?width=200 "grayling]]}}")
![bleak]]}}](https://commons.wikimedia.org/wiki/Special:FilePath/PSM V35 D070 Scale of bleak.jpg?width=200 "bleak]]}}")
![chub]]}}](https://commons.wikimedia.org/wiki/Special:FilePath/PSM V35 D069 Scale of chub.jpg?width=200 "chub]]}}")
![pike]]}}](https://commons.wikimedia.org/wiki/Special:FilePath/PSM V35 D075 Scale of pike.jpg?width=200 "pike]]}}")
![[[goby]]}}](https://commons.wikimedia.org/wiki/Special:FilePath/Study of Fishes-Fig 14.png?width=200 "[[goby]]}}")
![flathead]]}}](https://commons.wikimedia.org/wiki/Special:FilePath/Study of Fishes-Fig 13.png?width=200 "flathead]]}}")
![scat]]}}](https://commons.wikimedia.org/wiki/Special:FilePath/Study of Fishes-Fig 12.png?width=200 "scat]]}}")
![emperor]]}}](https://commons.wikimedia.org/wiki/Special:FilePath/Study of Fishes-Fig 15.png?width=200 "emperor]]}}")
![gudgeon]]}}](https://commons.wikimedia.org/wiki/Special:FilePath/PSM V35 D072 Scale of gudgeon.jpg?width=200 "gudgeon]]}}")
![spine]]s.](https://commons.wikimedia.org/wiki/Special:FilePath/FMIB 45807 Atopomycterus nichthemerus.jpeg?width=200 "spine]]s.")
![[[Pufferfish]] spines are not modified scales but are developed by an independent gene network.](https://commons.wikimedia.org/wiki/Special:FilePath/Tetrodon patoca Achilles 182.jpg?width=200 "[[Pufferfish]] spines are not modified scales but are developed by an independent gene network.")
![Mandarinfish]] lack scales and protect themselves with a layer of smelly and bitter slime.](https://commons.wikimedia.org/wiki/Special:FilePath/Synchiropus splendidus 2 Luc Viatour cropped.png?width=200 "Mandarinfish]] lack scales and protect themselves with a layer of smelly and bitter slime.")
![[[Boxfish]] have plates of ossified skin fused together to form a rigid shell.](https://commons.wikimedia.org/wiki/Special:FilePath/Lactoria fornasini1.jpg?width=200 "[[Boxfish]] have plates of ossified skin fused together to form a rigid shell.")
![[[Seahorse]]s have thin skin stretched over bony plates arranged in rings.](https://commons.wikimedia.org/wiki/Special:FilePath/Hippocampus guttulatus Achilles 174.jpg?width=200 "[[Seahorse]]s have thin skin stretched over bony plates arranged in rings.")
![Some [[filefish]] appear scaleless because their scales are so small.](https://commons.wikimedia.org/wiki/Special:FilePath/Cantherhines fronticinctus.jpg?width=200 "Some [[filefish]] appear scaleless because their scales are so small.")
![To casual examination [[tuna]] seem largely free of scales, but they are not.](https://commons.wikimedia.org/wiki/Special:FilePath/Bluefin-big.jpg?width=200 "To casual examination [[tuna]] seem largely free of scales, but they are not.")
Wikipédia
Chondrichthyes (; from Ancient Greek χόνδρος (khóndros) 'cartilage', and ἰχθύς (ikhthús) 'fish') is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or bony fishes, which have skeletons primarily composed of bone tissue. Chondrichthyes are jawed vertebrates with paired fins, paired nares, scales, and a heart with its chambers in series. Extant chondrichthyes range in size from the 10 cm (3.9 in) finless sleeper ray to the 10 m (32 ft) whale shark.
The class is divided into two subclasses: Elasmobranchii (sharks, rays, skates, and sawfish) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class).
Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.
