what is placoid , tenoid and cycloid scales ? explain .
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A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as well as possible hydrodynamic advantages. The term scale derives from the Old French "escale", meaning a shell pod or husk.[1]
Scales vary enormously in size, shape, structure, and extent, ranging from strong and rigid armour plates in fishes such as shrimpfishes and boxfishes, to microscopic or absent in fishes such as eels and anglerfishes. The morphology of a scale can be used to identify the species of fish it came from.
Most bony fishes are covered with the cycloid scales of salmon and carp, or the ctenoid scales of perch, or the ganoid scales of sturgeons and gars. Cartilaginous fishes (sharks and rays) are covered with placoid scales. Some species are covered instead by scutes, and others have no outer covering on part or all of the skin.
Fish scales are part of the fish's integumentary system, and are produced from the mesoderm layer of the dermis, which distinguishes them from reptile scales.[2][3] The same genes involved in tooth and hair development in mammals are also involved in scale development. The placoid scales of cartilaginous fishes are also called dermal denticles and are structurally homologous with vertebrate teeth. It has been suggested that the scales of bony fishes are similar in structure to teeth, but they probably originate from different tissue.[4] Most fish are also covered in a layer of mucus or slime which can protect against pathogens such as bacteria, fungi, and viruses, and reduce surface resistance when the fish swims
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what is placoid , tenoid and cycloid scales ? explain .
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PLACOID SCALES- found only in elasmobranch fishes. They have backwardly directed spines arising from a rounded basal plate embedded in dermis. Spines is made of enamel like basal plate of dentine like bony material. These scales are closely set together in skin giving it a sandpaper like quality.
CYCLOID SCALES- it is thin, flexible, translucent plates, has circular outline,thicker in centre, and marked with several concentric lines of growth which can be used to determine the age of fishes. They composed of a thin upper layer of bone and a lower layer of fibrous connective tissue. It is found in lung fishes, and several lower teleosts.
CTENOID SCALES- it is found in higher teleosts such as perch, sunfish,etc.. it's structure and arrangment are similar and more firmly attached and their exposed fre hind parts which are not overlapped, bears numerous small comblike teeth or spines these scales are dorsally ctenoid and ventrally cycloid.