Since Squeak version 2.5 there is no WorldMorph class anymore.
An instance of WorldMorph was the topmost morph which contained all other Morph instances. It kept a list of morphs which need stepping and sent them a stepping message when the scheduled time had come.
Nowadays a PasteUpMorph is used instead of a WorldMorph. With the message isWorldMorph you can check if it is meant beeing a World.