External covering is the outside appearance of an animal. Animals can have fur, feathers, hair, short hair, long hair, smooth hair, bristles, skin, thick skin, moist skin, dry skin, scales, waterproof scales, small scales, overlapping scales, spikes, hard shells, soft shells, smooth shells, rough shells, wool, or no covering at all.
Iridescence means shiny with many colours. Many animals have iridescent colours.
Iridescence is structural colour from refracted light (crystals), diffraction gratings (feathers or butterfly wings), thin film (oily surfaces), multiple layer interference (nacre in shells), or 3-D arrays of spheres (opals).
The word iridescence comes from iris, the Greek word for rainbow. An iridescent object has many tones of colour. Iridescence creates colour by splitting and reflecting light from different structures – which is why it is often called structural colour.