Jaw less fishes like Lampreys and hag fishes have thick skin. Many types of epidermal glandular cells are present which secrete a protective cuticle. In hag fishes multicellular slime glands produce large amount of mucous slime that covers the body surface. They protect the animal from external parasites. Thus hag fishes are called slime eels.
The skin of cartilaginous fishes like sharks is multilayered and contains mucous and sensory cells. The dermis contains bony placard scales called dentives. Dentives contain blood vessels and nerves and are similar to vertebrate teeth and grow throughout life. Like teeth once denticles reach maturity wear down and are lost. Denticles give cartilaginous fishes a sand paper texture. The skin of boy fishes contains scales composed of dermal bone.
A thin layer of dermal tissue overlaid by superficial epidermis normally covers scales. As scales are not shed, they grow at margins and over the lower surface. In many bony fishes growth lines are present that indicate the fish age. The dermis is richly supplied with capillary beds for respiration. Epidermis also contains many mucous glands. Mucus production helps to prevent bacterial and fungal infections and it reduces friction as the fish swims. Some species have granular glands which secrete on irritation or to some species poisonous alkaloid. Many Teliost (bony fishes) that live in deep aquatic habitats have photophores that facilitate species recognition or act like lures and warming signals.
The skin of Amphibians consists of stratified epidermis and dermis containing mucous glands, serous glands and pigmentation cells. Considering them phylogenetically they are transitional between aquatic and terrestrial vertebrates. The earliest amphibians were covered with dermal bone scales like their fish ancestor. The problems faced by amphibian due to terrestrial environment are (i) desiccations the damage caused by the ultra violet rays and (ii) physical abrasion. Evolution resulted in increased amount of Keratin production in the inner layers of skin which napped to protect the cells from UV rays. Mucus produced by mucus glands performs three functions (i) prevents desiccation (ii) facilitates gas exchange and (iii) makes the body slimy. Some poison glands are found in the dermis which produce unpleasant tasting or toxic fluid which acts as a predator deterent. Sensory nerves penetrate the epidermis as free nerve endings.
Skin of reptiles reflects their adaptation to their terrestrial mode of life. Outer epidermal layer (stralum corneum) is thick, lacks gland and is modified into Keratinized scales, suctes (thick scales) in turtles, beaks in turtles, rattles on snake and claws, plagues and spiny crests on most other reptiles. This layer resist abrasion, inhibits dehydration and protects the animals. During molting the older outer layer separates from new epidermis. Diffusion of fluids aids separation.
Skin of birds consists of outer epidermis of ectodermal in origin and inner dermis of mesodermal origin. Epidermis consists of outer epitrichuim, middle stratum corneum and an inner stratum germinativum or stratum Malipighium. Dermis consists of muscle fibres, nerve fibres, blood capillaries and connective tissue. Epidermis and Dermis and no pigmented cells but pigments occur on scales and feathers.
The skin of birds lacks sweet or other types of cutaneous glands except uropygial or preen gland. Skin of Mammals is thick and covered with hair. Skin consist of two main layers (1) the epidermis, ectodermal in origin and dermis which mesodermal in origin. From skin arise hairs, claws, nails and glands. Epidermis consists of outer stratum corneum. This layer has hard Keratin. Below it is stratum lucidum followed by thin layer of stratum gramulosum. Innermost layer of epidermis is stratum malpighium or stratum germinativum made of columnar cells. At the base of epidermis are pigment cells. Dermis is thick, tough, flexible and elastic covering whole body. Dermis is formed of connective tissue fibres, unstriped muscles, blood capillaries, nerves and fat cells. Some of the epidermal glands like sweet glands, sebaceous glands etc are found in dermis.
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