Looked at the University today and found these articles that might have some anotomical pictures in them:
Induced sexual Maturation of European Eel...
June 30 2003
Localization of Vascular type ATPase.... swim bladder
J. Exper Biol
There were actually about 226 different articles on Anguilla anguilla I just did a search under the Biological Science Index. Though, for anatomical diagrams and what not it would be best to look for articles from the earlier part of the last century or earlier.
I found these books under a search for Eels:
(I have not had a chance to look at them yet)
Eels, A naturel and unnatural History
Myers, George Sprague
Four new genera and ten new speices fo eel from the Pac coast.
Wade, Charles Barkley
Two new genera and five new species of apodal fishes...
I hope to have a chance to peruse these books and others tomorrow or this week sometime. But, this should be a start