Phylum Chordata: Class Mammalia

Class Mammalia

Persian Leopard

Welcome to the world of mammals! There are around 4,500 living species in this class. They are characterized by many attributes: vertebrate, endotherm, fur/hair, breathing air, producing milk, and a four-chambered heart.
Mammals use their hair to stay warm and to protect their skin from the environment. Their fur also plays a role in keeping their body temperature stable. This next characteristic is what mammals were named after--mammary glands. These glands are found in female mammals, and are used to create milk to feed their young.

Their circulatory system is well-developed. It has four chambers--two atria and two ventricles. It also has two separate loops forming pulmonary (heart to the lungs) and systemic (heart to the body) circulation. In pulmonary circulation, blood is pumped into the right atrium, and then into to the right ventricle. After that, it goes to the lungs where it gets oxygen. Then systemic circulation occurs. Blood travels back to the heart in the left atrium, then, to the left ventricle. Finally, it goes to the rest of the body through blood vessels.

Mammals have large brains compared to their body size. The cerebrum is the most prominent. The outer layer of the cerebrum is called the cerebral cortex. This part of the brain is in charge of a mammal's thinking, learning abilities, and other complex behaviors. Other parts of the brain include the olfactory bulb, optic lobe, cerebellum, and mendulla oblongata.

There are three groups of mammals: monotremes, marsupials, and placental mammals.

Duckbill Platypus

Monotremes have two reptile-like features. They lay eggs that hatch outside the mother's body, and their digestive, urinary, and reproductive systems all have one opening--the cloaca. However, females still have mammary glands that produce milk. There are only three living species of monotremes: duckbill platypuses, and two species of spiny anteaters.

Koalas

Marsupials give live births and complete their development inside the mother's pouch. After an egg is fertilized, it forms into an embryo inside the mother. It is born very early and finds the mother's nipples. They are located in an external pouch called the marsupium. It stays there for several months continuing development and drinking milk from the mother. Examples of marsupials include kangaroos, koalas, and wombats.

Placental mammals are named after the key characteristic that separates them from other mammal groups--the placenta. The placenta is used to exchange nutrients, gases, and wastes between the mother and embryo. Because of this, the embryo can stay inside the mother for a longer period of time. After birth, mothers nurse their young with milk. Examples of placental mammals include raccoons, dogs, elephants, dolphins, horses, and humans.

Below, are pictures of placental mammals: