Completed another two “worlds” today, having spent ages and ages trying to get past the Starfish level. It’s just like one of the levels in Super Monkey Ball 2 on the Gamecube (as, in fact, many levels are), but for some reason, it’s $hlmun times harder on the DS game. I think I might be due to the gap in the “star” being narrower, and the star stopping for a shorter time.
Anyway, with that level out of the way, I managed to clear the two worlds mentioned, and have two others unlocked too. Tried (and failed) them both. Bah.