I've often noticed that when it comes to individual episodes of animated series, sometimes they have no actual writing credit on them, instead either using "Story by" or even no such credit whatsoever. As it turns out, there's actually two different kind of animated series: script-driven, and board-driven. So, I'm assuming when they lack writing credits, those would be the board-driven shows, where rather than using a script for the voice actors to work from, they just use storyboards.