The human body has different types of glands: � Exocrine glands � Endocrine glands and � Mixed type glands The secretion of endocrine glands are called hormones. Scientific definition of hormone is: Hormones are non - nutrient chemicals which acts as intercellular messangers and are produced in trace amounts. The new definition covers a number of new molecules in addition to the hormones secreted by the organised endocrine glands such as: Steroids, proteins and protein derivatives. Invertebrates have very simple endocrine system. Whereas in vertebrates there are many chemicals that acts as hormones and provides coordination.