An endocrine organ usually associated with the thyroid gland. Embryologically, the glandular primordia are formed in the endoderm of the third and fourth pharyngeal pouches and are associated with the similarly derived primordia of the thymus. There may be from one to three pairs of the small glands present in individuals of the various vertebrate classes, although two pairs appear most frequently. They are characteristically within, on, or near the thyroid gland. In response to lowered serum calcium concentration, a hormone is produced which promotes bone destruction and inhibits the phosphorus-conserving activity of the kidneys.