Which one of the following is not a ductless gland?

An endocrine gland is a ductless gland: it releases its secretion (a hormone) directly into the bloodstream. An exocrine gland, in contrast, releases its secretion through a duct onto a surface or into a cavity such as the mouth.

Checking each gland against this rule:

• Adrenal gland - an endocrine gland; it releases hormones such as adrenaline and cortisol directly into the blood, with no duct.

• Pituitary gland - an endocrine gland; it releases hormones such as growth hormone directly into the blood, with no duct.

• Salivary gland - an exocrine gland; it releases saliva through a duct into the mouth.

• Thyroid gland - an endocrine gland; it releases thyroxine directly into the blood, with no duct.

Cross-check: the standard ductless (endocrine) gland list includes the pituitary, thyroid, adrenal, and parathyroid glands and the islets of the pancreas. The salivary glands are grouped with the sweat and sebaceous glands as ducted (exocrine) glands, not in this list.

Since the adrenal, pituitary, and thyroid glands are all ductless (endocrine) glands, and only the salivary gland releases its secretion through a duct, the salivary gland is the one that is not a ductless gland.