General Overview: The class Mammalia includes around 5,499 species, such as bats, tigers, lions, elephants, whales, rats, rabbits, horses, sheep, and monkeys. Mammals inhabit a wide variety of habitats, including forests, grasslands, mountains, deserts, polar regions, and caves. Some mammals, such as bats, are adapted for flying, while others, like whales, are adapted to live in water.
Body Structure: The body is divided into head, neck, trunk, and tail. Mammary glands are a key characteristic, present in all mammals, with well-developed glands in females for nourishing offspring. The skin contains sweat glands, sebaceous glands, and is covered with hairs, fur, nails, claws, horns, or hooves. External ears (pinnae) are present, a distinguishing feature of mammals. The endoskeleton is fully ossified, with skull and vertebrae connected by intervertebral discs.
Locomotion: Mammals have two pairs of pentadactyl limbs for various forms of locomotion such as walking, running, climbing, burrowing, flying, or swimming. In some mammals, like whales, forelimbs are modified into flippers for swimming. Digits end with horny nails, claws, or hooves.
Digestive System: The digestive system is complete, with various types of teeth (incisors, canines, premolars, molars) for specialized feeding habits.
Respiratory System: Mammals have spongy, elastic lungs. A larynx is present for sound production.
Circulatory System: Mammals have a four-chambered heart with two auricles and two ventricles. RBCs are oval, biconcave, and enucleated (without a nucleus).
Excretory System: Excretory organs are kidneys, which remove nitrogenous waste as urea (making mammals ureotelic).
Nervous System: The brain is highly developed, with a large, convoluted cerebrum and cerebellum. Mammals have 12 pairs of cranial nerves.
Sense Organs: The ears have three regions: external, middle, and internal. External ears, or pinnae, are fleshy and protect the opening. Eyes are protected by movable eyelids, with the upper eyelid being the primary mover.
Reproduction: Mammals are unisexual and exhibit sexual dimorphism (differences between males and females). Fertilization is internal. Most mammals are viviparous (give birth to live young), except for prototherians (like the platypus), which lay eggs. Embryo development occurs in the uterus. The placenta connects the fetus to the uterine wall, facilitating nutrient and oxygen exchange. Prototherians do not form a placenta, and development is direct without a larval stage. Parental care is highly developed, and mammals nourish their young with milk from the mammary glands.
Amniotic Development: Mammals are amniotes, meaning the embryo develops with four extra-embryonic membranes: amnion, chorion, yolk sac, and allantois.