• Largest group of mammals, the placentals, have a placenta which feeds the offspring during gestation. 
• Before birth, the young are nourished through a placenta. 
• The placenta is a specialized embryonic organ that is attached the mother's uterus and delivers oxygen and nutrients to the young.

Skeletal system

• The majority of mammals have seven cervical vertebrae bones in the neck. Example: bats, giraffes, whales, and humans. 
• The few exceptions include the manatee and the two-toed sloth, which have only six cervical vertebrae, and the three-toed sloth with nine cervical vertebrae.

Respiratory system

• In air-breathing vertebrates, respiration takes place in the respiratory organs called lungs. 
• The passage of air into the lungs to supply the body with oxygen is known as inhalation, and the passage of air out of the lungs to expel carbon dioxide is known as exhalation; this process is collectively called breathing or ventilation. 
• In humans and other mammals, the anatomical features of the respiratory system include include trachea, bronchi,bronchioles, lungs, and diaphragm. 
• Molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous external environment and the blood. 
• This exchange process occurs in the alveoli air sacs in the lungs