Fun fact: Freud once spent a summer in Italy dissecting 400 eels in a futile search for their testes.
>In 1874 Dr. Syrski had announced the most recent solution. He had discovered a small lobed organ and described it as the testes of the eel. Carl Friedrich Claus, chief of the Institute for Comparative Anatomy in Vienna, assigned to his student Freud the task of checking Syrski's observations. Freud dissected 400 eels, finding the Syrski organ in many of them. On microscopic examination he found the histological structure of the organ such that it could well be an immature form of the testes, though he found no definite evidence that this was the case. [3]
Modern researchers propose that Syrski's organ is an optional precursor to the testes [4].
[0] For the German readers among us who wish to settle this debate decisively https://archive.org/stream/sitzungsberichte75kais#page/n367/...
[1] https://www.historytoday.com/archive/natural-histories/sexua... https://biodiversity.utexas.edu/news/entry/sigmund-and-his-e...
[2] http://nautil.us/issue/88/love--sex/eels-dont-have-sex-until... (an interview with the author of TFA) https://twitter.com/greenleejw/status/1205524393490833408 (This is where I originally picked up the fact, strongly recommend you follow if you have even a passing interest in medieval eel facts)
[3] https://www.jstor.org/stable/26301225
[4] https://www.researchgate.net/profile/Benjamin_Geffroy2/publi... (pdf)
> My investigations now lead me to confirm Syrski's data almost throughout. However, the histological examination of the organ does not make it possible for me to agree decisively with the opinion that this is the testis of the eel or to refute it with certain reasons.