Nautiluses are super cool!! They’re the oldest living subclass of the cephalopods, and really haven’t changed all that much since the late triassic! Their ancestors were called ammonites which appeared waayyyy back in the devonian period (419.2–358.9 million years ago). I had a whole post on ammonites a long time ago which you can find here: http://squidscientistas.tumblr.com/post/124712468719/firstly-you-guys-are-awesome-and-i-love-this
Nautilus shells are composed is many gas chambers and as it gets bigger the nautilus will close off one of the chambers and move its body into the next one.