Although it was not applied until well after the Goetia was written, the top point sigil is the sign of the asteroid Ceres. I do not know if it was ever applied toward the goddess Ceres or where it came from... it may be a mere coincidence.
The central design looks, ironically, like a swastika.
The designs are all cramped into place, but each is different and balanced with its complementary shape. It is probable that they are names or that they represent some spiritual powers, but it is possible that they are only aesthetic additions. Whoever writes the grimoire decides what the sigils will be, and they may have simply decided to draw sigils in the pentacle. I suppose not, though, and I would love to hear what your research into the sigils produces.
--------------------
|