Exocrine glands have two structural classifications, unicellular (one cell layer) and multicellular (many cell layers). Goblet cells are unicellular exocrine glands; so named for their shape, these glands secrete mucus and are found in the epithelial lining of the respiratory, urinary, digestive, and reproductive systems. Multicellular exocrine glands are classified by their shape of secretory parts and by the arrangement of their ducts. A gland with one duct is a "simple," whereas a gland with a branched duct is a "compound" gland. The secretory portions of simple glands can be straight tubular, coiled tubular, acinar, or alveolar (flask-like). The secretory portions of compound glands can be tubular, acinar, or a combination: tubulo-acinar.