The term " integument " is applied to any outer covering of an animal. Basically, it means the skin, although many scientists describe the exoskeleton of arthropods as an integument. An exoskeleton is a coating of hard protein type substances that entierly cover the outside of the animal. It provides a place for muscle attachment. The vertebrate skeleton is internal and muscles attach from the outside. This permits larger growth of the animal. Vertebrate animals have developed some very interesting excrescences, or projections of the skin, some of which are well-known and easily oberved by most people. They are scales, feathers, and hair. Although these characteristics are common among the most vertebrates, they are not all that common in the animal world as a whole.

The primary function of the excrescences is protection and insulation. Claws, hooves, and nails are other types of excrescences...