Summary: A system of chemical communication among cells in vertebrate animals. The classical vertebrate endocrine system (Fig. 1) consists of a group of discrete glands that secrete unique products (hormones) into the bloodstream. These products travel in the blood to distant sites or targets, where they cause specific physiological responses. Thus, endocrine glands differ from exocrine glands because they lack ducts and deliver their secretions in the bloodstream. The classical definition of an endocrine system is also harder to apply with the discovery of scattered cells rather than discrete glands that act as endocrine organs, of endocrine cells that affect themselves (autocrine effect) or nearby targets (paracrine effect) by diffusion through extracellular fluids rather than the bloodstream, and of neurons that secrete hormones (neurosecretion). All of these mechanisms, however, allow for chemical intercellular communication and can be considered part of the endocrine system