FISH SKIN
All fishes being aquatic possess more or less similarly adapted skin. They possess a multilayered epidermis of living cells that is covered on the surface with a layer of cuticle. Fishes lack a typical stratum corneum made of dead and cornified cells as found in tetrapods.
Multicellular glands also occur in some fishes, e.g. electric organs of eels and electric rays, and luminescent glands in deep sea fishes. Mucous glands are all unicellular.
Important modifications of skin are dermal scales that cover the body for protection. 
Kinds of scales.The following types of scales are found in living and extinct fishes:
Cosmoid scales. They were found in ancient crossopterygian fishes of Devonian period and were many layers thick and strong but not flexible. Apparently ancient crossopterygii, with thick and hard scales covering the body, were not active and fast swimming animals.
Ganoid scales. They are thick rhomboidal scales having the surface coated with a hard enamel-like material called ganoin and lacunae (pits) and canaliculae (fine canals) forming the surface sculpture.
Cycloid and Ctenoid scales. These scales are found in modern teleosts and are thin, strong and extremely flexible. They are large, oval in shape and made of isopedine and fibrous tissue.
Placoid scales. They are characteristics of cartilaginous fishes (Chondrichthys) and are hard and microscopic in size.
Evolutionary modifications in scales of fishes
Ostracoderms and placoderms, which were ancestors of all modern fishes, had their bodies covered with dermal bony armour for protection from predators. These fishes were sluggish in movement and were bottom feeders of detritus. To attain flexibility, the bony armour gave way to cosmoid scales which were still like thick bony plates covering the body. In ganoid fishes, the spongy bone of cosmoid scales was lost. Cycloid and ctenoid scales of modern teleosts are very thin and flexible and consist of only isopedine and fibrous tissue. Thus modern teleosts are agile and quick swimmers.
AMPHIBIAN SKIN
Amphibian skin shows adaptation to terrestrial life but the animals have to return back to water for reproduction and hence the skin also demonstrate certain adaptation to aquatic mode of life. On land a major problem faced by the animal is of desiccation and strong solar radiation. There are no scales on the skin of modern amphibians, although ancestral labyrinthodont amphibians had dermal scales on their skin for protection. Also, the scaly protection gone, amphibians evolved mulicellular poison glands to deter potential predators.
Amphibian skin is the simplest skin having no particular modifications as found in higher groups of vertebrates but it has typical structure of terrestrial skin that is subsequently carried through to reptiles, birds and mammals.
REPTILIAN SKIN
Reptiles are completely independent of water. They do not have to go back to water for breeding as they are capable of laying a large cleioid, shelled egg that can develop and hatch on land. Reptiles are adapted to life in arid climate and have very few glands. Then there are dermal plates on the body of crocodiles and turtles.
Exoskeleton of Chelonia (tortoises and turtles) consists of a dorsal convex carapace and a ventral flat plastron which are attached by ligaments on the lateral sides. Outer surface of the carapace and plastron consists of epidermal plates called scutes while the inner side is made of dermal plates.
Carapace is fused with the vertebral column and attached to the plastron on the sides.
BIRD SKIN
Birds possess thin skin that is loosely attached to body to allow free movement of wings during flight. Basic structure is of typical tetrapod skin. Feathers are characteristic modifications of bird skin, which not only cover the entire body but also help in flight. Glands are absent in bird skin except the uropygial glands near the tail that secrete oil for preening feathers. Modifications of skin include epidermal scales on the legs and feet, claws, beak, spur, comb etc.
Contour feathers.Also called quill feathers, they are hard feathers that cover the entire body and form the contour or outline of the body by which we identify the bird. Ventral side of the shaft has a longitudinal groove called umbilical groove. Barbs on either side of the shaft area arranged parallel to each other and in such a way that they form a flat Vane that has aerodynamic shape.
Down feathers. Also called plumules, they are soft feathers that serve as insulation between the hard contour feathers and the delicate skin. Powder down is a term used to identify such feathers in adults whereas nestling down appear in chicks.
Hair feathers. They are hair-like thin and long feathers, also called filoplumes or pin feathers and are scattered all over the body. They are especially modified to guard nostrils, ear openings and serve as eye lashes. They are made of long thin hair-like shaft to the top of which are attached a bunch of thin barbs and barbules.
Development of feather
Feathers grown on certain certain tracts on the skin called feather tracts or pterylae and feathers do not grow on the areas termed as apterylae or apteria but the growth is dense and hence the entire body is covered with feathers. Precocial chicks as in fowl and other game birds are covered with nestling down when they hatch from the eggs but altricial chicks such as sparrow and pigeon hatch naked and development of feather can be observed during the growth of chicks. First indication of the growth of feathers on skin is the appearance of pimple-like elevations called feather primordium that changes into feather follicle in the middle of which the feather grows. The body of feather is produced by multiplication of epidermal cells assisted by feather papilla.
MAMMALIAN SKIN
Mammalian skin is thick, tough, glandular and covered with hairs. Dermis is much thicker than epidermis but general structure exhibits typical terrestrial skin characters. Epidermis in some areas is very thick and can be differentiated into several layers. Dermis is thick and composed of collagenous connective tissue which makes the entire skin tough and fibrous. Mammalian skin has abundance of glands, particularly sudoriferous or sweat glands and Sebaceous glands which are oil secreting glands attached to the hair follicle. Scent glands or Hedonic glands found in musk deer and civets are also modifies epidermal glands.
Mammalian skin becomes sensory owing to the presence of large number of free nerve endings and specialised receptors such as pacinian corpuscles, meissner’s corpuscles, tactile discs, Krause’s end bulbs etc. Also the base of hair follicle is supplied with nerve, making the hair sensory. Carnivores, rodents and many other mammals possess specialised hairs on the upper lip called whiskers, which are sensitive to air currents, touch and vibrations.
Development of hair
Hairs grow on the skin in the embryonic stage and continue to grow throughout life. When a hair has to grow, epidermal cells in that area multiply fast forming a mass of cells called hair primordium that carries a dermal papilla at the base. Keratinization of epidermal cells inside the hair follicle produce the hair root and shaft that grows out of skin. Length of the hair is determined by the wear and tear to which the tip of the hair is exposed and the thickness of the hair shaft.
Development of Mammary glands
Mammary glands are apocrine compound tubulo-acinar glands that specialise in secreting nourishing fluid called milk that is sucked from teats by the young ones of mammals. Development of mammary glands is completed during embryonic stage in both male and female sexes. The position and number of nipple formation along the milk line varies in different species, e.g. one pair of axillary nipples are formed in flying lemurs, thoracic in primates, bats and sirenians, one or two pairs of inguinal in ungulates.
When the mammary gland has to develop, epidermal cells of milk line in that particular location start multiplying fast forming a mass of mammary tissue that grows and branches to form a mass of considerable size. The development of mammary glands stops at this stage in both sexes and further development takes place only in female at the time of puberty. Unlike in hair or feather, development of mammary glands does not need the assistance of dermal papilla.
Monotremes possess primitive mammary glands which are perhaps modified sweat glands that got attached to the hair follicles.
Horns in mammals
Mammals sport one pair of horns on the head which are organs and defence and offence and sometimes serve as secondary sexual organs of male. The following five types of horns are found in mammals:
- True horns or Hollow horns, as found in family Bovidae that includes cattle.
- Pronghorns, found in pronghorn antelope.
- Antlers, as found in deer family Cervidae.
- Knob horns or Giraffe horns, found in giraffes.
- Hair horns or Keratin fibre horns, found in rhinoceroses.
TRUE HORNS. These horns are found in cattle, sheep, goat and antelopes. They may be straight, curved or spiralling as in antelopes and sheep and are made of bony core and a keratinized epidermal sheath.
PRONGHORNS. They are found in pronghorn antelope, Antilocapra americana that inhabits North American plains and also in Saiga antelope of Russia.
ANTLERS. They are found in the males of family Cervidae and are secondary sex organs meant to attract females as well as to intimidate or fight the rival males. These horns are made of bony core that grows from the frontal bones of the skull and is covered with living velvety skin.
KNOB HORNS. They are found in giraffes and okapi in both sexes and are permanent structures. Structurally they are similar to the antlers with bony core covered with velvety skin.
HAIR HORNS. Found in rhinoceros, they are made of specialised hairs or keratinised fibres that get agglutinated to form a pointed and hard horn.
OTHER MODIFICATIONS OF MAMMALIAN SKIN
- Many mammals have epidermal scales as on the feet and tail of rats and mice.
- There are dermal bony plates on the body of armadillo (order Edentata).
- Ischial or Lilac callosities on monkey’s buttocks are cushion-like structures on which the animal sits on hard branches and rocks.
- Kneel pads on the legs of camels are meant for absorbing shock when the animals kneels and sits on the ground.
- Tori are epidermal pads on fingers, palms and soles of feet of primates that help in firm grasping branches of trees while climbing.
- Baleens are horny epidermal plates numbering about 370 that hang from the roof of the oral cavity of toothless whales. They are used to filter planktons such as krill on which these giants of the sea feed.
- Claws, nails and hooves are also cornified structures derived from the epidermis of skin. They are made of two types of cornified tissue, the outer hard and shiny tissue called unguis, under which lies a softer tissue called subunguis which lies in contact with the living tissue.