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Endocrine glands and hormones


Endocrine glands lack ducts and are hence, called ductless glands. Their
secretions are called hormones. The classical definition of hormone as a
chemical produced by endocrine glands and released into the blood and
transported to a distantly located target organ has current scientific
definition as follows: Hormones are non-nutrient chemicals which
act as intercellular messengers 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. Invertebrates
possess very simple endocrine systems with few hormones whereas a large
number of chemicals act as hormones and provide coordination in the
vertebrates.
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