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The integument or the skin forms the external covering of a vertebrate. It is continuous with the mucous membrane lining the mouth, rectum, nostrils, eyelids, and the opening of urinogential ducts. The skin comes in contact with the environment, and the type of environment whether terrestrial or equatic, influences the character of the skin. The vertebrate skin differs from that of invertebrates in having two layers, an outer epidermis derived from the ectoderm, an and inner dermis or corium formed from the mesenchyme of the dermatomes, the relative amount of the two layers veries with the environment.
Function of the Intigumentary System
Derivatives of the Integument
Both layers of integument have given reise to various types of derivates. The epidermis gives rise to integumentary glands, epidermal scales, horns, tips of digits, different corneal structures, feathers, and hairs. The dermis forms dermal scales of fishes and of some reptiles.
A. Epidermal glands
According to function the epidermal glands of vertebrates are of the following types:
After the evolutionary loss of dermal scales of fishes, amniotes developed an entirely new type of scale derived from the epidermis. The skin of vertebrates is rarely naked, it is usually provided with protective scales, bony plates, feathers or hairs.
C. Corneal structures
Some epidermal scales of the tail of rattle-snake are modified to form a rattle, it consists of a series of old dried scales.
These are found in ungulate mammals only, five types of horns are recognized
In amniota the distal ends of digits have claws, nails or hoofs formed from the horny layer of the epidermis . They grow parallel to the surface of the skin and are built on the same plan.
Feathers are found only in birds and are modified reptilian scales, they are formed from the epidermis in which the stratum corneum is highly specialized. Feathers are light, strong, elastic, waterproof and show many colours due to pigmentss and structural arrangement. They form a protective covering, regulate body temperature, and support the body in flight.
Hair is found only in mammals, it protects at an acute angle from the skin. Hair covers the entire integument in most, but in others only traces are left, such as whales have only a few coarse hairs on the snout.