Class Mammalia

Hair and Mammary Glands
The most important function of hair in mammals is that the hair insulates them against the cold. The mammary gland is located in the breasts of females that is responsible for the production of milk. Both males and females have glandular tissue within the breasts; however, in females the glandular tissue begins to develop after puberty.

Circulatory System
The blood flow is directed in two circuits: one through the lungs and back to the heart, and the other throughout the rest of the body and its organs.

Brain
The brain size usually increases with body size in animals. As you grow older, you will notice that everything in your body grows up with you, including your brain. An interesting fact about your brain is that once you are born, your brain remains as its newborn stage, so it's almost impossible to remember what happen when you were a baby. As you grow older, memories and languages continue to pile up, so your brain grows larger. However, the oldest memories can be forgotten over time.

Classes
Monotremes: Monotremes are mammals that can lay eggs rather than giving birth to live young. However, they still nourish their young with milk and are warm-blooded. Some examples of monotremes are the echidna and platypus. Marsupials: These animals give birth to incomplete offspring. Because of this, the newborns are typically carried and suckled in a pouch on the mother's belly. For instance, kangaroos carry joeys (baby kangaroos) in their pouches until they're old enough. Others include the opossum, Tasmania devil, or sugar glider. Placental Mammals: Unlike monotremes or marsupials, this is a widely diverse group. Their offspring exchange food and wastes with their mother through blood. This is why humans are born with a belly button, which are the remains of the umbilical cord, a cord used to distribute nutrients to your stomach. There is a wide range of placental mammals like cats, dogs, humans, or primates.