Hi Sven - it can be difficult to remove edges like that, basically you have to get the surfaces for the faces on either side of such an edge to be merged together into one single surface in order for the edge to be removed.

If the other side does not have that little edge, then there is a shortcut though since you have a symmetrical part there - delete all the sphere pieces making up that one testicle and then select the opposite half and use Transform > Mirror to mirror it over, and then use Edit > Join to glue it back together.

- Michael