In this article we will discuss about the role of skin in vertebrates and invertebrates with the help of suitable diagrams.
The skin or integument (La. inlegwnentum) protects the animals from dangers in the environment and invasion of bacteria, parasites, predators and ultraviolet radiations. In no case, the animal is totally isolated from its surroundings and in cases where the exoskeleton is hard—as in crabs and certain molluscs there are openings through which contact between the body and the environment is maintained.
In many animals the integument has a regulatory role and exchange of water, gases and ions takes place across it. Receptors and external glands are present in the skin. It also helps in heat regulation. In most animals, pigment cells present in the skin imparts a particular colour to the animal.
Invertebrate Integument:
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The integument is usually made up of a basement membrane-supported simple, columnar epithelium, termed epidermis (Gr. epi = upon + dertna = skin). An exposed epidermis may contain many ciliated and gland cells. In some cases an overlying non-cellular cuticle (annelids, arthropods) or a shell (molluscs) is secreted by the gland- cells in the epithelium, where the latter is termed hypodermis.
Vertebrate Integument:
The vertebrate integument consist of an epidermis of stratified epithelium supported by an underlying dermis made up of a thick layer of connective tissue. In some animals, the epithelial cells synthesize and accumulate keratin (Gr. Keras = horn) a protective and water insoluble layer.
In fishes, the epidermis contains mucous cells and multicellular glands. Their secretion covers the body surface with a layer of mucus and helps in reduction of friction with water and also prevents attachment of parasites and predators. In some fishes, horny dermal scales (Fig. 7.9) develop, the posterior parts of which overlap the scales of the next row and protect the fish. Spines and plates are modified scales.
Amphibian skin is basically similar to that of the fishes, except that it does not synthesizes keratin and may contain poison glands. In terrestrial vertebrates, the skin is greatly modified to reduce the loss of body water and to make it more protective.
The epidermis is highly keratinized and forms a distinct horny layer, the stratum corneum. This layer is quite thick in reptiles and forms horny scales and plates (Fig. 7.10A).
The feathers of birds are basically elongated and flattened horny scales (Fig. 7.10B). In mammals, a different type of epidermal derivative is hair, the follicle of which extends to the dermis. A layer of air entrapped in the feathers and hairs acts as an insulator against heat loss.
The skin of reptiles and birds is practically without glands except a few scent (odoriferous) glands. On the other hand, a large number of glands are present in mammals. The sebaceous (L. sebum = tallow) glands, the sweat glands are ingrowths of epidermis into the dermis.