Mammalia — hair and milk

This is the class of animals that nourish their young on milk. Mammals such as humans, mice, squirrels, monkeys, whales, dolphins, sambar, deer and buffalo belong to it.

Figure to be added

Figure 13.32 — several species of Mammalia: macaque, fruit bat and dolphin.

They bear a skin covered with hair, sunk into follicles in the skin.
Mammary glands (producing milk), sweat glands (producing sweat) and sebaceous glands (producing sebum) are present.
They bear external ear pinnae.
The heart has four chambers — two atria and two ventricles.
A complete double circulation occurs.
They bear biconcave red blood cells without nuclei.
They are homoiothermic.
The testes lie outside the body.
Fertilisation is internal. Embryonic membranes develop and a placenta is present.

Four mammalian orders

Order Primates — tree shrew, squirrel, monkey, orangutan, gorilla, chimpanzee, human
Order Chiroptera — insect bat, fruit bat
Order Cetacea — whale, dolphin
Order Artiodactyla — sambar, deer, buffalo

The chamber count tells the story

Fishes 2, amphibians 3, reptiles 3 (with an incompletely divided ventricle), birds and mammals 4. Four chambers means oxygenated and deoxygenated blood never mix at all — which is what supplies the energy a warm-blooded animal needs.