A skeleton is a structural frame that supports an animal body.
The skeletal system of mammals and other vertebrates is broadly divisible functionally into axial and appendicular portions.
There are two major types of skeletons: solid and fluid. Solid skeletons can be internal, called an endoskeleton, or external, called an exoskeleton, and may be further classified as pliant (elastic/movable) or rigid (hard/non-movable).
Fluid skeletons are always internal.
The presence of hair or fur.
A four-chambered heart.