Glands can be either simple or compound, with simple glands containing one duct and compound glands containing many ducts. Glands are also classified as either exocrine or endocrine, with exocrine glands secreting substances through ducts and endocrine glands secreting directly into the bloodstream without ducts. Additionally, glands can be categorized based on their secretions, such as mucous secreting glands which produce mucus or protiosecreting glands which secrete proteins.