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INTRODUCTION, Fish are aquatic vertebrates. They make up more than half of all vertebrate, species. They are especially important in the study of vertebrate evolution, because several important vertebrate traits evolved in fish. Fish show great, diversity in body size. They range in length from about 8 millimeters (0.3, inches) to 16 meters (about 53 feet). Most are ectothermic and covered with, scales. Scales protect fish from predators and parasites and reduce friction with, the water. Multiple, overlapping scales provide a flexible covering that allows, fish to move easily while swimming., , STRUCTURE OF FISH, , MKL

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STRUCTURE OF FISH INTEGUMENT, ● The integument is composed of the skin and skin derivatives,and includes, scales in fishes., ● The integument forms an external protective structure parallel to the, internal endoskeleton and serves as the boundary between the fish and the, external environment., ● The structure of the skin in fishes is similar to that of other vertebrates,, with two main layers:an outer epidermis and an inner dermis., , MKL

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Epidermis, ● The epidermis is ectodermal in origin., ● In lampreys and higher vertebrates, the epidermis is stratified., ● The upper layer is caller Stratum cornium ,it is present in some fishes but, generally absent ., ● The lowest layer is the stratum germinativum, composed of columnar, cells . It is the generating layer that gives rise to new cells., ● In hagfishes, lampreys, and bony fishes, thereis an outer thin film of non, cellular dead cuticle (Whitear1970)., ● The outer part of the epidermis in terrestrial vertebrates is the stratum, corneum, which is composed of dead, horny, keratinized squamous cells, that form hair and feathers., , Dermis, ● The inner dermis contains blood vessels, nerves, sense organs, and, connective tissue., ● It is derived from embryonic mesenchyme of mesodermal origin., ● It is composed of fibro elastic and nonelastic collagenous connective, tissue with relatively few cells., ● Dermal layers include an upper,relatively thin layer of loose cells, the, stratum laxum(or stratum spongiosum) and a lower, compact thick layer,, the stratum compactum, ● In adult fishes, the dermis is much thicker than the epidermis., ● The thickness of the integument depends on the thickness of the dermis, ● .Scaleless species, such as catfishes of the genus Ictalurus,have relatively, thick, leathery skin., ● The Ocean Sunfish (Mola) has the skin reinforced by a hard cartilage, layer,50–75 mm thick., ● Snail fishes (Liparis, Liparidae) have a transparent jellylike substance up, to 25 mm thick in their dermis., FISH SKIN COMPOSITION, , MKL

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● The chemical composition of fish skin is poorly studied,but some, generalizations can be made, ● There is less water in fish skin than in fish muscle and similar amounts, of protein., ● The main protein in skin is collagen, which is why fish skin has been, used to manufacture glue., ● The chief minerals in fish skin are phosphorus,potassium, and calcium, (Van Oosten 1957), , EPIDERMAL DERIVATIVES, 1. Club Cell ● A large number of round oval or elongated unicellular gland of large size, are found in epidermis of many species .they are called Club cell or Giant, Cell and are present in varrasius auratus , barbus, catla catla ,mrigala, labeo Rohita etc according to some author club cell secrete alarm, substance which causes fight reaction in many species, , MKL

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2. Mucus Cell, ● Mucous glands secrete a protein called mucin, which with water forms, the substance known as mucus; this slimy material serves to lubricate the, body, thus lessening friction and aiding locomotion in swimming animals, 3. Photophore cells, ● In fishes, pigment is produced in branched cells known as, chromatophores, which can be found in both epidermis and dermis., ● Rapid colour change, by which some fishes can adapt to a change of, background, is brought about by redistribution of the pigment within the, cell boundaries. Slow, long-term changes involve alterations in the, numbers of cells or in the amount of pigment they contain., MKL

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4. Poison gland, ● Other structures in the skin of fishes include epidermal venom glands, associated with spines on fins (weeverfishes, Trachinidae; mad tom, catfishes, Noturus), opercles (venomous toadfishes, Thalassophryninae),, and the tail (stingrays, Dasyatidae)., 5. Mucus gland cell, ● Form coccun in infavourable condition, ● It is generally seen in teleost fishes, , DERMAL DERIVATIVES, , 1. Cosmoid Scales:, ● Found in Placoderms (extinct) as plates, and also typical of the Lobe, Finned Fishes or Sarcopterygii, (Choanichthyes)., ● Extinct fish had scales of enamel, cosmine and bone with pulp cavities., ● Modern ones, like Coelocanth and the lung fish have calcified fibers so, this type of scale is almost extinct. No specimens available., 2. Placoid Scales:, ● See bioplastic mounts and dogfish slides., ● Made of enamel (epidermal) and the dermal derivatives, dentine and bone, with a pulp core., ● Typical of cartilaginous fishes., ● Placoid scales are responsible for the rough feeling of dogfish skin., , MKL

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4. Teleost (bony fish) scales, ● These are thin scales of dermal bone. They have a thin covering of, epidermal tissue over them., ● It is derived by reduction (loss) of parts of a ganoid scale. There are two, types depending on their shape., 1. Cycloid Scales, ● See bioplastic mounts and slides. A round ended scscale, 2. Ctenoid Scales:, ● See bioplastic mounts and slides., MKL

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● A comb shaped end is characteristic of this scale type., , FUNCTION OF FISH SKIN, ● Among the functions of the skin are mechanical protection, ● production of mucus by epidermal mucous cells., ● Photophores, which produce bioluminescence, develop from the, germinative layer of the epidermis, ● .Color is due to chromatophores, which are modified dermal cells, containing pigmen, , MKL

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Homework, ● More Function of Fish Skin, , 😁, , MKL