Exocrine glands can also be classified by their secretion mechanism.
Merocrine glands - Glands that secrete substances by the process of exocytosis (fusion of cytoplasmic vesicles with the plasma membrane resulting in the release of their contents into the extracellular space without compromising the integrity of the cell membrane). Sweat glands (eccrine sweat glands) and salivary glands are merocrine in nature. The lining of the respiratory and digestive tracts contain "goblet cells" - glandular epithelia that synthesize and secrete a complex glycoprotein called mucus. Mucus provides a protective function as well as serving as a lubricant.
Apocrine glands - Glands that secrete by shedding the apical portion of their cytoplasm into a duct (e.g., mammary glands). Sweat glands in the axillae, perianal region, and external genitalia are also apocrine in nature.
Holocrine glands - Glands that secrete by shedding entire cells from the lining of a duct (e.g., sebaceous glands).