I can't handle lowercase letters. The images are missing I guess. What is so confusing about the traditional notation? The only things which can be confusing are xyzMES, but you didn't use them here yet. ;)

Use the same size for all worlds. Right now, there are these scroll bars and it is not really cool that you have to use them instead of seeing everything ;)

In the original, you can move a tetromino when it hits the bottom for a short time. If you still didn't turn it in yet, you could try to implement that.

The box popping up when clicking on a creature should be displaced on top of everything else. If you need a new idea of what you could do next... you could display the number of creatures of each type over time in a diagram and show it after the simulation is over.

Then you should try out Traffic Simulation 2 (http://www.greenfoot.org/scenarios/13356) and Traffic Simulation 3 (http://www.greenfoot.org/scenarios/15117). They are much better than this. ;)