Mammalia
Mammalia is the vertebrate class that includes humans, whales, bats, and other animals whose living members produce milk and typically have hair.
Mammary gland: A milk-producing organ found in mammals, formed from modified skin glands. Milk production is the defining reproductive feature of living mammals.
Lactation: The production and secretion of milk by mammary glands to nourish offspring. It is the process through which mammalian mothers provide milk.
Synapsid: A member of the vertebrate lineage that includes mammals and their extinct relatives. Mammals evolved within Synapsida, a lineage distinct from the one leading to reptiles and birds.
Reptile: An amniote vertebrate in the reptile lineage, including turtles, lizards, snakes, crocodilians, and birds. Reptiles and mammals are distinct amniote lineages with different evolutionary histories.
Hair: A keratin-based filament growing from the skin of mammals. Hair is a characteristic mammalian trait, though it varies greatly in form and abundance.
Placenta: An organ that enables exchange of nutrients, gases, and wastes between a developing embryo and its parent. Placental development supports pregnancy in most living mammal lineages.
Therapsid: A major group of synapsids that includes mammals and many extinct mammal-like forms. Therapsids include close evolutionary precursors of mammals.
Bird: A feathered, beaked vertebrate in the living dinosaur lineage. Birds, like mammals, are endothermic, but they evolved this trait independently.
Endothermy: The ability to generate internal heat and regulate body temperature through metabolism. Mammals maintain relatively stable body temperatures through internal heat production.
Monotreme: A mammal in the egg-laying groups that include the platypus and echidnas. Monotremes show that mammalian reproduction does not always involve live birth.