The epididymis (Greek epi "auf" and didymos "doppelt", "Zwilling") is a smaller structure of the two testicles. They are mainly formed by the 4 to 6 m long epididymis duct (Ductus epididymidis).
The Semen is produced in the testicles. It is then transported to the epididymis. They consist of small channels that are 4 - 5 meters long.
Here the sperm mature completely, are stored and enriched with a secretion that inhibits their movement, so that they do not consume the energy stored in them prematurely. Non-functional sperm become phagocytised.
The epididymides are located at the rear and upper part of the testicles and have an iroquoise-like appearance.
The easiest way to feel the epididymis is after a hot bath or shower when the scrotum skin is relaxed. Sometimes when boys feel these swellings, they're afraid it might be cancer. However, they are a normal part of the male anatomy.
https://de.wikipedia.org/wiki/Nebenhoden (Wikipedia CC-by-sa-3.0)