Strictly speaking, there is a difference between the epididymis, which is an organ on the surface of the testis; and the duct which fills its entire volume, the ductus epididymidis. The one encloses the other. The duct has a recognizable wall, and the organ has its own boundaries. But this is mere hair-splitting: for all intents and purposes the duct and the organ are inseparable. It's common practice to refer to the "epididymis," which is not only easier to spell, it's easier to say.