Strange.I will have to keep up on this post to see if it's dividing or if it's the All terrible Such and Such!!! Joking!!! Only Joking!!! Don't get worried.
I've had several Nems split during my time in the hobby. I don't believe that hole is a mouth. In my experience, when a Nem splits, a portion of its mouth goes with it. When it heals, a new mouth is formed.