Fish demo" tutorial problem / question

So I'm trying to be a little creative with my implementation of movement along the y-axis ... I'm having trouble getting the onWorldLimit function to actually trigger when the fish hits the top / bottom limits.


Below's what I've got. As near as I can tell, the "top" and "bottom" cases aren't getting hit at all. The fish just up and goes right off the top/bottom of the screen.

// Tutorial 5
function FishClass::onWorldLimit(%this, %mode, %limit)
{
switch$ (%limit)
{
case "left":
%this.setLinearVelocityX(20);
%this.setFlipX(false);

case "right":
%this.setLinearVelocityX(-20);
%this.setFlipX(true);
%this.setLinearVelocityY(getRandom(-20, 20));

case "top":
%this.setLinearVelocityY(-20);

case "bottom":
%this.setLinearVelocityY(20);
}
}