integumentary system of vertebrates
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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