With regards to your problem - do you mean you want to switch from one world to another? At the moment this isn't possible via two different subclasses, the way you have to do this is by removing all of the elements from your world and then adding in the ones for the next level. It shouldn't be too difficult to do, just use removeObject() and addObject() in world to set things up how you need.

On my first go, the animals all crowded around the water tank and it was next to impossible to get any water out without them drinking it. Also, sometimes I seemed to jump to the top of the screen when hit by the animals - is this meant to happen? Nice game though :-)

Hmm, I would've thought a number like 30 would've been more reasonable - that was always the benchmark figure I was taught in terms of beyond that you can't really notice a difference. Nice idea though!