[ Ancient arthropods were able to grow to ginormous sizes for two main reasons. First, oxygen was much more concentrated in the atmosphere 400 to 500 million years ago, making up 30% of Earth’s atmosphere instead of 21% today. Arthropods rely on air flowing through their bodies in order to take in oxygen; they don’t have lungs like us. The extra oxygen basically served as added fuel for growth. Second, and more simply, there weren’t really any larger predators around to cull their numbers. Call it evolutionary luck. ]
Concerning oceans the ocean speciaes arthropods have gill like structures.
http://en.wikipedia.org/wiki/Gill
Crustaceans, molluscs, and some aquatic insects have tufted gills or plate-like structures on the surfaces of their bodies.
Third, there were no pots of boiling water, no nutcrackers, and of course, without mammals, there was no clarified butter.
It wasn’t just extra oxygen. The key that is hardly ever mentioned is that the atmospheric pressure must have been much higher as well. High enough, in fact, that the atmosphere’s pressure allowed extremely large creatures to live on land, due to buoyancy, like they still do below the oceans.
If you just had more oxygen in the atmosphere, but the same pressure, then creatures would not have grown much larger than they do today. They would have reached their full size faster, but they couldn’t grow much larger.