Gianluca, David, I have replaced the bladder on the Basso and the Suzuki and yes they are glued at the perimeter of the bladder leaving the center unglued. I have attached a link where I discuss changing the bladder on the Hohner Basso. Unfortunately I did not take pictures of the Suzuki replacement only the Hohner Basso. All three are very similar in construction. this is a step by step process you can follow.
Hope this helps you.