Body Structure
Mammals have a skeleton which supports muscles and ligaments. All mammals possess mammary glands. Hair made of keratin is present in at least one stage of their lives.

Vertebrae and Limbs

They have a backbone made up of vertebrae. The backbone can be divided into five parts: cervical, thoracic, lumbar, sacral, and caudal. The atlas and axis vertebrae in the cervical region allows mammals to move their heads. The thoracic vertebrae attaches the ribs, which protect important organs. The lumbar vertebrae attaches the legs. Sacral vertebrae supports the pelvis, and the caudal are the tail bones. Some mammals such as dolphins have the lumbar and sacral vertebrae joined together. Most have four limbs ending with phalanges (fingers and toes). 

In the lower jaw, there is one bone on either side. All other animals have more than one bone.
In the middle ear, there are three bones. One of the bones once used to be a bone in the lower jaw.
The skull encloses the well developed brain. The neocortex is a region in the brain that is special to mammals. It deals with sensory preception, reasoning, thinking, and language. All mammals have a hard palate that separates the nose and the mouth. Most mammals have 2 sets of teeth of different shapes. Most have 7 neck bones. Exceptions are the manatee and the 2 and 3 toed sloth.

Body Cavity
A body cavity is a space full of fluid in which organs are held. It protects the organs and allows them to develop. In mammals, the body cavity is called a Coelom. The thorax and abdominal cavities are separated by a diaphragm.