Skin of fishes, amphibians, reptiles, birds and mammals

 Skin of fishes:

Jawless fishes have thick skin. It is composed of epidermal glandular cells. Some of these glandular cells secrete protective cuticle. Multicellular slime glands are present in hagfishes. These glands produce large amount of mucous slime. This slime covers body surface. It protects from external parasites. Skin of cartilaginous fishes is multilayered. It contains mucous and sensory cells. Dermis contains small placoid scales. These scales are called denticles. Denticles contain blood vessels and nerves. Their structure is similar to vertebrate teeth. It protects the skin surface. Denticles project above surface of skin, so they give sandpaper texture. Skin of bony fishes contains scales. These scales are composed of dermal bones. Scale has thin layer of dermal tissue. It is covered by superficial epidermis. Scales are not shed, so they grow at margins and over the lower surface. Growth lines are present on scales, these are useful in determining the age. Skin of bony fishes is permeable. It functions in gas exchange. Dermis is richly supplied with capillaries. These capillaries help in respiration. Epidermis contains many mucous glands. Mucous prevents bacterial and fungal infections. It reduces friction as fish swims. Some species have granular glands. It secretes irritating poisonous alkaloid. Many teleost live in deep aquatic habitats. They have photophores. Photophores help in species recognition. It acts like lures. It gives warning signals.

Skin of amphibians:

Amphibian skin consists of stratified(composed of many layers)epidermis. Its dermis contains mucous and serous glands. It also contains pigmentation cells. Amphibians are transitional state between aquatic and terrestrial vertebrates. Earliest amphibians were covered by dermal bone scales. These were present in their fish ancestors. Keratin is tough impermeable protein that protects skin in physically abrasive rigorous terrestrial environment. Keratin production increases during amphibian evolution. It is present in outer layer of skin cells. It protects from ultraviolet light. Mucous glands produce mucous. It prevents from desiccation. It keeps skin moist. Thus helps in gas exchange. It makes body slimy. Thus these animals are escaped from predators. Poisonous glands are present within dermis of some amphibians. It produces unpleasant taste or toxic fluid. This fluid acts as predator deterrent. Sensory nerves penetrate epidermis. It has free nerve endings. Warts of toads are specialized sensory structures. These warts contain many sensory cells. 

Skin of reptiles: 

Reptiles are fully adapted to terrestrial environment. Their skin is also adapted for terrestrial environment. Their outer layer of epidermis is called stratum corneum. Stratum corneum is thick keratinized layer. It lacks glands. It forms keratinized scales and scutes. Scutes are thick scales in snakes and turtles. It forms beaks in turtles and rattles on snakes. It resists abrasion. It inhibits dehydration. It protects like a suit of armor. Shedding or molting takes place in skin of many reptiles for example snakes and lizards. Old outer layer separates from newly formed epidermis. Fluid is diffused between layers. It helps in this separation. 

Skin of birds:

Skin of birds shows many reptilian features. It has no epidermal glands. Only epidermal gland of birds is preen gland. Epidermis is thin and it is only two or three cell layers thick. Outer layer is keratinized and it is often quite soft. Epidermis contains feathers. Feathers are most prominent part of epidermis. Feathers are derived from scales of reptilian ancestors. These are most complex keratinized epidermal structures. Dermis of birds is similar to dermis of reptiles. It contains blood, lymphatic vessels and nerves. It also contains sensory bodies. These bodies are epidermally derived. Air spaces are part of avian respiratory system. These air spaces extend into dermis. Air spaces are involved in thermal regulation. Smooth muscle fibers are present in dermis. They have complex arrangement. They are associated with feathers. They control position of feathers. Feather position is important in thermal regulation, flying and behavior. Aquatic birds may also have fat deposits in hypodermal layer. It stores energy and insulate the body. 

Skin of mammals:

Hairs are present on skin of mammals. Different types of epidermal glands are present. They have highly stratified and cornified epidermis. Their dermis is thicker than epidermis. Epidermis is composed of stratified squamous epithelium. It consists of several layers. These layers have different types of cells. Outer dead layer of cells is called stratum corneum. Rapid cell divisions take place in deepest layer of epidermis. It pushes cells toward surface of skin. These cells die and become keratinized. Keratinized cells form outer layer. This layer is called stratum corneum. Keratin is insoluble in water. Therefore stratum corneum prevents dehydration. It defends body from toxic substances and microorganisms. Prevention from dehydration is important adaptation in mammals. It helps mammals to adapt in terrestrial environment. Stratum malpighian is present below stratum corneum. It is composed of living cells. These cells continuously divide. Dermis is thick layer. It contains blood vessels, lymphatic vessels, nerve endings, small muscles and glands. Hypodermis is present below dermis. Its composition is different from other vertebrate classes. Blood vessels pass from hypodermis and enter into dermis. Blood vessels are absent in epidermis. Hypodermis consists of loose connective tissues, adipose tissues and skeletal muscles. Connective tissues form ground tissues in dermis. Adipose tissues store energy in the form of fat. They also provide insulation in cold environment. Skeletal muscles allow skin to move independently.