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respiratory question

I've been following the string on gills to lungs, and it's quite interesting. 
We just started respiratory in my biology class and we got a sheet from my 
teacher label "Evolution of Respiratory Systems." It just starts with 
sponges, I think, and talks about their lack of a system, then moves up to 
annelids and other sorts up to birds and mammals. I thought I might bring in 
some of the strings from the list but I'm in a 9th grade biology class and 
the string sort of builds from previous posts. I was wondering if someone 
could give a brief description of how lungs accually developed that I could 
bring in for my class, or, if I understand the string right, there's not a 
certain way that we're sure it happened, so maybe something about the 
theories. I'll been in some other the other posts to just to show my teacher.