Integument, cutis or simply skin is the outermost covering of the body. It is the first organ of the body that is visible when we encounter an animal and helps us in identification of the type of organism. Although simple in appearance, skin is a complex organ that performs many functions in the body and hence is sometimes aptly called “The Jack of all trades”, which means it carries out different functions but is “master of none” that is the function is not so vital that it may threaten the life of the animal. The important functions of the skin are outlined below which also emphasize the importance of skin as an organ of body.
Protection. Skin being the outermost covering of body is the first to encounter the harsh environmental conditions and is modified in different ways to protect the body. Aquatic animals which possess a skin made of all living cells secrete a layer of cuticle and mucous over the skin for protection but the terrestrial animals encounter a much more harsher environment and hence exhibit tough layer of dead and keratinized cells on the surface and different modifications to enhance protection depending upon the needs. For example, animals like crocodiles, turtles and armadillo have a dermal bony covering over the body to protect them from predators. Hedge hogs and porcupines have evolved spines on the skin as a means of defence.
Pigmentation in the skin provides protection by camouflage. Another type of protection provided by the skin to the body is by possessing poison glands as in amphibians.
Exteroception. Skin functions as a sensory organ as the entire surface area of the body is covered by skin. Free nerve endings are scattered in the dermis of the entire skin. There are specialized cutaneous receptors, such as pacinian corpuscles, meisser’s corpuscles, tactile discs etc. which convey to the brain senses of touch, pressure, heat and cold.
Respiration. Cutaneous respiration occurs in amphibians when they are living an aquatic life. Many reptiles, such as sea turtles and sea snakes can also carry out some amount of respiration through skin.
Excretion. Many toxic substances simply evaporate through the surface of skin. Sweat glands also serve as excretory organs because they are the first organs that excrete toxins out of the body.
Salt-water regulation.Maintenance of salt-water ratio in blood is of utmost importance that is carried out by sweat glands in mammals, by excreting excess salts and some amount of urea out of body.
Temperature control.In summers mammals excrete sweat that spreads on the surface of skin to cool the body and in winter fur keeps the body warm. Whales, sea lions, walruses and polar bears deposit a thick layer of fat under the skin, which is called blubber for insulating the body from outside subzero temperature.
Food storage. Fat deposits in the adipose tissue of skin functions as stored nutrition apart from conservation of body heat in almost all animals. During lean period when food is scanty, the stored fat is utilised to sustain life. Migratory birds, hibernating animals also use stored fat as food.
Nourishment. Skin synthesizes vitamin D in the presence of sunlight and calcium, when dehydrocholesterol is converted into vitamin D.
Locomotion.All kinds of locomotory organs are assisted to various degrees by the skin in carrying out their functions. Modified skin called patagium that stretches between fingers of fore limbs in bats. Skin is modified in the limbs of all animals for walking, running, swimming, flying etc.
Brood pouch. Brood pouch is a bag of skin that is usually attached to the belly in marsupials. Some other animals such as sea horse, pipe fish and some amphibians develop brood pouches for keeping their eggs or juveniles.
Sexual selection. Male competition to fertilise female is an essential component of sexual selection, for which males develop various kinds of secondary sexual characters that are modifications of skin. Long and colourful feathers of male birds, anters in deer family and mane of lions are all contributions of skin in sexual identification and attraction.
No other organ of body performs as many functions as the skin, as described above and hence skin is sometimes termed as “The jack of all trades”.
STRUCTURE OF SKIN
Chordate skin is characterised by having two layers, the outer epidermis which is derived from the embryonic ectoderm and the inner dermis, a derivative of mesoderm. Non-chordate skin lacks dermis except in echinoderms.
Protochordates are marine animals and show aquatic adaptations in the skin. Amphioxus skin is made of the outer epidermis and an inner dermis but epidermis is composed of single layer of living cells which must be protected on the surface by a layer of cuticle and a basement membrane that attaches epidermal cells with dermal tissue. Dermis is composed of loose connective tissue and lacks melanocytes.
SKIN OF CYCLOSTOMATA
Cyclostomes are also marine animals which are parasitic or flesh-feeders. They lack scales on the skin and are dark in colour but the larval skin is ciliated. Epidermis is made of all living cells but is thick and multilayered and is covered with a layer of cuticle for protection. There are unicellular mucous glands in epidermis.
Dermis of cyclostomes is composed of collagenous connective tissue, which is tough and fibrous tissue that provides strength to the skin. Thus, the cyclostome skin is simple, without many modifications as seen in more advanced groups.