The skin or integument is often overlooked when we think of different systems, but it is the largest mass-wise in the body and without it vertebrates can't survive. This is why burn victims are at such risk. Water balance - most obvious in amphibians, but gills are used in water balance in fishes, in terrestrial vertebrates it inhibits water loss. Thermoregulation - Changes in blood flow to the skin can increase or decrease heat exchange. Color is used in thermoregulation by lizards and roadrunners and many others.
Skin glands produce pheromones to aid in courtship and territoriality.
The superficial epidermis comes from ectoderm and the deeper dermis from mesoderm. This is true in all verts. The thickness and subdivisions of these vary throughout the subphylum. If we make a generic vertebrate we can look closer at the layers. Starting with the epidermis and going from dermal to external we have 2 layers.