Gland, any structure of animals, plants, or insects that produces chemical secretions or excretions. Glands are classified by shape, such as tubular and saccular, or saclike, and by structure, such as simple and compound. Types of the simple tubular and the simple saccular glands are, respectively, the sweat and the sebaceous glands (see Skin). The kidney is a compound tubular gland, and the tear-producing glands are compound saccular (see Eye). The so-called lymph glands are erroneously named and are in reality nodes (see Lymphatic System). “Swollen glands” are actually infected lymph nodes.

Glands are of two principal types: (1) those of internal secretion, called endocrine, and (2) those of external secretion, called exocrine. Some glands such as the pancreas produce both internal and external secretions. Because endocrine glands produce and release hormones (see Hormone) directly into the bloodstream without passing through a canal, they are called ductless. For the functions and diseases of endocrine glands, see Endocrine System.

In animals, insects, and plants, exocrine glands secrete chemical substances for a variety of purposes. In plants, they produce water, protective sticky fluids, and nectars. The materials for the eggs of birds, the shells of mussels, the cocoons of caterpillars and silkworms, the webs of spiders, and the wax of honeycombs are other examples of exocrine secretions.