Dear Mansour Ebrahimi,

I think there are not a global comprehensive diagnostic key. The work present in this link is a good start point and can be useful for a preliminary study on morphology and determination of high taxonomic level (at least for family and maybe genus).

http://aquaticcommons.org/2694/1/tr121opt.pdf

Best regards,

Hudson

I confirm. There is, unfortunately, no modern key for the Monogenea.

However, several books about monogeneans are available. I suggest to begin with the classic book by Yamaguti (1963)

Yamaguti S. 1963. Systema Helminthum Volume IV Monogenea and Aspidocotylea: John Wiley & Sons.

I agree with Profesor Justine. Maybe another book (Yamaguti, S. 1968. Monogenetic trematodes of Hawaiian fishes) will be helpful

I am grateful to you all for the participation and guidance. Your comments were consequent and I found what I wanted.