The function of tunica externa is to

The tunica externa (New Latin "outer coat"), also known as the tunica adventitia (New Latin "additional coat")(or adventitia for short), is the outermost tunica (layer) of a blood vessel, surrounding the tunica media. It is mainly composed of collagen and, in arteries, is supported by external elastic lamina.

To protect and/or reinforce the blood vessels  

D.) domain, kingdom, phylum, class, order, family, genus then species

Both of them have a nucleus, golgi apparatus, cell membrane, smooth and rough endoplasmic reticulum, lysosome, and ribosomes.