Different hormones can often be made from the same precursor molecule by cleaving it with a different enzyme. Most of the similarities among the endocrine systems of crustaceans, arthropods, and vertebrates are examples of convergent evolution . More than fifty different hormones are secreted. The type of endocrine product is determined by which tissue layer a gland originated in. The endocrine system uses cycles and negative feedback to regulate physiological functions.
Negative feedback regulates the secretion of almost every hormone.
Cycles of secretion maintain physiological and homeostatic control. These cycles can range from hours to months in duration. Image from W.H. Freeman and Sinauer Associates, used by permission. The endocrine system acts by releasing hormones that in turn trigger actions in specific target cells . Receptors on target cell membranes bind only to one type of hormone.