The spine has an external covering of enamel-like, hard, transparent material called vitrodentine. This is followed by a layer of dentine enclosing a pulp cavity from which several branching dentine tubules radiate in different directions. The centre of the basal plate is perforated by an aperture to provide entrance to the blood vessels and nerves from the dermis. The placoid scales are closely set in skin but do not overlap each other and giving a sand paper-like quality to the skin.
The cycloid scales are thin, flexible, transluscent plates, rather circular in outline, thicker in the centre and marked with several concentric lines of growth which can be used for determining the age of the fish. They are found in a large number of teleostean fishes having soft rayed fins, such as Labeo, Catla, Barbus, Cirrhina, etc.
The central part of the scale is called the focus and is the first part to develop. In many species, oblique grooves or radii run from the focus towards the margin of the scale. They form a protective covering over the skin and project diagonally in an imbricating pattern. The part of the scale which is exposed to view in situ condition posterior area generally shows less distinct ridges or circuli and chromatophores are also sometimes attached to it.
The anterior area lies embedded in the skin. The ctenoid scales are also circular and can be distinguished from the cycloid scales by having a more or less serrated free edge. Moreover, several spines are present on the surface of the posterior area of the scale. These scales are characteristic of modem higher teleosteans such as perch, sunfish, etc.
They are more firmly attached and their exposed free hind parts which are not overlapped, bear numerous small comb-like teeth or spines. Intermediate types between cycloid and ctenoid scales are also found on different parts of the body. Certain fishes, such as flounders, may bear both types, ctenoid scales dorsally and cycloid scales ventrally.
Some fishes are totally scaleless or naked such as Torpedo electric-ray and catfishes. In eels, they are minute and deeply embedded in dermis. In some fishes chimaeras they become localised. Jaw teeth in the sharks are regarded as modified scales. The teeth forming the saw of Pristis and the sting-ray are modifications of the scales.
The scales at the base of tail in the sturgeon fish, Acanthurus, are modified to forming sharp cutting blades. In coffer fish, Ostracion, the scales form polygonal bony plates articulating with one another to form a rigid protective box. In the globe-fish, Tetrodon and the porcupine fish, Diodon the scales are elongated to form spines for protection.
The jointed fin-rays, leptotrichia of the bony fishes are also considered to be modified scales. In the basking shark, Cetorhinus, the scales become modified to form gill-rakers. The scales along the lateral line of the fish become perforated to provide an exit for the canal. In the sea-horse. Hippocampus, and the pipe-fish, Syngnathus, scales are fused to form protective bony rings around the body. Scales form a protective covering of exoskeleton on the body.
The structure of scales is useful in broad classification of the group. Thus, among the fishes, placoid scales are characteristic of Chondrichthyes, ganoid scales are found in the primitive bony fishes, while the cycloid and ctenoid scales are found in the higher teleosts. Sometimes, the scales are useful in separating orders and families. Thus, siluroid fishes are scaleless and can be distinguished from cyprinoids. Scales form an important source of information about fossil fishes and in classifying kitchen middens of extinct men.
Scales are useful in studying the food habits of piscivorous animals. Thus a placoid scales become partially dermal and partial epidermal from the basal side. The thorns protrude towards the top of the skin while the base plate is hidden in the dermis. Placoid scales become larger and change to form shark teeth. In vertebrates, teeth erupt in the same way and have a comparable structure.
Therefore, placid scales and vertebrates teeth are also considered to be coexistent. No such live fish can be found. The scales were present in the bodies of some Ostracoderms, placoderms, and extinct sarcopterygian. There are four levels in this scale. On the outside of the scales is an enamel-like thin and hard vitrodentin layer, below this layer is a hard and non-cellular cosmin layer, on the inside of the cosmic layer is the isopedine layer, which consists of the duct and the ossicular layer.
The growth of these scales occurs towards the edge. It does not grow here because there are no living cells at the bottom. In the case of lung fish, the original cosmoid structure has changed to cycloid scales. Ganoid scales consist of thick, usually diamond-shaped plates. The roof also has tile-like scales attached side by side to form a bony covering.
In some cases, the scales overlap. One of these fish is called ganoid fish. Pelioniscoid ganoid scales exist in the bichir Polypterus. These scales consist of an enamel-like ganoid layer on the outside, a dentin-like cosmin layer on the middle, and a bony isopodine layer on the inside.
Lepidostoid ganoid scales are present in Lepisosteus. The scales have ganoin on the outside and isopedine on the inside. This scale increases in all directions.
Such scales are thin, piercing. It does not have enameloid and dentinal layer. This type of scales is found in most living bony fish Osteichthyes. It is of two types, namely:. Such scales are found in lungfish, some Holosteans and non-teleostean such as carp Cypriniformes , hilsa Clupeiformes and cod Gadiformes. Such scales consist of round plates. The center of this scales is called the focus. Many concentric circular growth lines can be seen from the focus.
The upper part of these growth lines is made up of thin bony and the lower part is made up of fibrous connective tissue. At certain times of the year, especially in winter, the growth of fish decreases as the growth of scales decreases, resulting in the formation of ridge or circuli in the scales. The large circulus is known as annulus which is formed annually. In some species, many radiuses are seen near the focus. The scales overlap each other.
The scales are located in a small pocket on the front of the dermis and the rear part is exposed. Such scales can be seen in modern advanced teleostean fish such as perch Perciformes , sunfish, etc. The shape, texture and decoration are exactly like cycloid scales. However, there are small thorns cteni in the open back side. This is why this type of scales is called ctenoid scales. These scales are more firmly attached. Some fish such as Jew fish Johnius flat fish Cynoglossus , flounder fish have both cycloid and ctenoid types.
These fish have ctenoid scales on the surface and cycloid scales on the numerical side. Different types of modifications are observed in fish scales.
Some fish such as electric fish, catfish Siluriformes , eel Anguiliformes have small scales that are placed in the dermis. Some fish have scales transformed into different organs. Shark jaw teeth, dorsal spines of dogfish Squalus and chimerid Chimera , tail spines of stingrays Dasyatidae , saw teeth of saw sharks Pristis , gill racker of basking sharks Cetorhinus , etc are conversion of placoid scales.
The lancet of the sergeon fish Acanthurus is formed by converting the tinoid scales. The abdominal scutes of the herring Clupeidae are transformed into cycloid scales. Lepidotricia of the fins and dermal bones are the modifications of bony ridge. Sharks also have a thick fibrous dermis that part of the skin which is below the epidermis which supports the scales and helps protect the animal.
Other cartilaginous fish that swim like sharks, such as the Guitar-fish Rhinobatidae and the Saw-fish Pristidae also have a complete covering of denticles. However, those species that are dorso-ventrally flattened — such as the rays — tend to have many fewer denticles. In the Skates Rajidae they are scattered in patches across the pectorals and on the head. In the Eagle Rays Myliobatidae they are very few in number and in the Electric Rays Torpedinidae they are absent, except in the modified form of the tail spines.
The Devil-fishes Mobulidae have none at all. In their ancient form, they had four layers. One of dense bone, one of spongy bone, one of dentine and one of enamel. The scales of the remaining bony fishes have only two layers, a calcified one and a fibrous one. Ganoid scales are derived from Cosmoid scales and are the evolutionary older style. They are found on Bichirs, Gar-fish, Sturgeons and Reedfish. The most common form of fish scale is the elasmoid scale.
It is the thin plate that you find on most fishes. However there are intermediate forms and these two terms are really just adjectives that represent the extremes of a continuum. The scales of a fish may be all of one kind perch have ctenoid scales while herrings, minnows and trout all have cycloid scales or both type of scales may be found on the same fish. Sea Perches, Epinephelus sp. Not all fish have scales, some species such the Sun-fish Mola mola and the Siluroidei Naked Catfish have none at all.
Other species, like the Common Eel appear to have no scales, but they really have microscopic scales deeply embedded in their dermis. Even in those fish that have scales, they do not always cover the whole body. Thus fish that swim quickly, or that live in fast flowing waters Trout, Tuna etc , tend to have small scales. While fish that swim slowly in slow moving waters, tend to have larger scales, i. In some S. American Catfishes, the scales have become modified into bony plates to make armour.
This is also the case in Sea horses and Pipe fish. In these cases the added protection is paid for by reduced flexibility and speed.
0コメント