What are the 4 wheels and 9 doors in Snp 1.29?

What are the four wheels and nine doors mentioned in the sutta below? Why is it "born from a bog"? What is the meaning of the sutta? From the Four Wheels Sutta (Sutta Nipata 1.29) : > “Having four wheels and nine doors, > Filled up and bound with greed, > Born from a bog, O great hero! > How does one escape from it?” > > “Having cut the thong and the strap, > Having cut off evil desire and greed, > Having drawn out craving with its root: > Thus one escapes from it.”