Epidermal glands are formed from the Malpighian layer of the epidermis, according to their structure they are unicellular, tubular or alveolar glands.
(a) Unicellular glands are single modified cells found among other epithelial cells, they are present in amphioxus, cyclostomes, fishes and larvae of amphibians. Unicellular glands are known as mucous cells or goblet cells. They secrete mucin which forms slimy or sticky mucous on coming in contact with water which lubricates the surface of the body.
Other unicellular glands are granular cells and large beaker cells of cyclostomes and fishes, they also secrete mucous.
(b) Tubular glands are multicellular tubes of uniform diameter formed as ingrowths of the malpighian layer into the dermis, glands of Moll on the margin of the human eyelids. Tubular glands may become coiled, e.g., sweat glands of mammals, or they may divide into many tubules which are then called compound tubular glands, e.g., mammary glands of monotremes, and gastric glands.
c) Alveolar or saccular glands are multicellular downgrowths of the Malpighian layer into the dermis having a tubular duct whose terminal parts form a rounded expansion to become flask-shaped, e.g., mucous and poison glands of amphibians. Alveolar glands may branch into many lobules which finally open into a common duct, they are then called compound alveolar glands, e.g., mammary glands of eutherians, and salivary glands.
According to function the epidermal glands of vertebrates are of the following types:
1) Mucous glands may be unicellular or multicellular, among the unicellular glands are mucous gland cells, granular cells and beaker cells of amphioxus, cyclostomes and fishes, they secrete mucus which keeps the skin moist and slippery, mucus cells and granular cells lie near the surface, but the beaker cells lie more deeply and extend from the Malpighian layer to the surface.
2) Luminescent organs or photophores are found in longitudinal rows near the ventral side of the body in those fishes which live in deep sea where no light penetrates. Each photophore is a group of epidermal cells lying in the dermis, it has a lower layer of glandular cells below which is a layer of reflecting pigment, the upper layer of cells forms a lens. The glandular cells produce phosphorescent light which is transmitted to the outside by other cells, the light helps to attract the prey of deep sea fishes.
3) Multicellular mucous glands are alveolar structures found in some fishes and in amphibians where they occur all over the surface of the body they produce mucus for lubricating the skin and in amphibians they keep the skin moist to aid in respiration. Amphibians also have alveolar poison glands which are larger but less numerous than mucous glands, in toads masses of poison glands form parotid glands behind the head. The secretion of poison glands and used as defence. Some tubular glands are found on the feet and suctorial discs of tree frogs which aid in climbing.
4) Femoral glands are found in male lizards below the thighs in a row from the knee to the cloaca, they secrete a sticky substance which hardens into short spines that are used for holding the female during copulation.
5) Uropygial glands is one of the few integumentary gland found in birds forming a prominent swelling just above the tail or uropygium, they secrete an oil which is odoriferous during sexual activity.
6) Sweat glands or sudorific glands are long coiled tubular glands, are abundant in the skin of most mammals. They are never unicellular. The lower coiled part is embedded in the dermis and is surrounded by blood capillaries. Sweat is secreted by sweat glands continuously. Sweat contains a large amount of water having dissolved salts of sodium, potassium and urea. Sweat removes some metabolic wastes and regulates the body temperature by evaporation. Sweat glands are not uniformly distributed; in man they are more numerous on palms, soles and arm pits; in cats, rats, and mice they are confined to the soles of the feet; in rabbit they are around the lips; in bats on the sides of the head; in hippopotamus they are found only on the pinnae. In some mammals the secretion of sweat glands is red-coloured. Cerumimous glands in the external ear passages of mammals are modified sweat glands, they secrete a waxy substances which combines with the secretion of sebaceous glands to form earwax which catches dust.
7) Sebaceous glands are branched alveolar glands opening into hair follicles of mammals and also independently around the genital organs, tip of nose, and edges of the lips. Sebaceous glands secrete an oil to lubricate the hairs and the oil also covers the skin with a film of oily coating. The oily secretion of sebaceous glands contain waxes, fatty acids, and cholestrerol. Sebaceous glands are absent in Manis (Pangolin), Sirenia and Cetacea.
8) Scent glands have a structure which shows that they may be either modified sebaceous glands or sweat glands, their secretion is an allurement to the opposite sex. Scent glands are located in the deer family on the head near the eyes, carnivores have scent glands around the anus, pigs and goats have scent glands between their toes.
9) Mammary glands are characteristics of mammals, they secrete milk generally in the females for nourishment of the young. In monotremes both sexes may secrete milk, this condition is called gynaecomastism which is not a normal condition. Mammary glands of monotremes are compound tubular glands, in other mammals they are compound alveolar. Mammary glands of monotremes have no nipples, they open into pits on the surface of the skin, and the young ones obtain milk by licking tufts of hairs. In others the mammary glands open by their ducts into a nipple or into a false nipple or teat. The nipple is a raised outgrowth of the breast. Mammary glands along with fat form integumentary swellings called mammae or breasts. The number and location of mammae varies in different mammals, the number ranges from two in many mammals to 25 in opossum.