If you store the position of on what lilypad it is only in the critter, how would the critter know how many neighbouring critters it has?
Embedding the information of the critters on the Lilypads instead of in their own process is possible. However, the main reason to split them into two, is because of a separation of concerns. If either of these things grows in complexity (say, we want to have special lilypads that only turtles can walk on), it can do so without directly affecting the other. Also, consider what happens when one of the things crashes: If a Lilypad-embedded-with-critter-info crashes, that critter is lost. If a Lilypad crashes whose critter is in a separate process, this process can wait for a new Lilypad to appear at its location.
Conversely, if a Lilypad-embedded-with-critter-info crashes while executing its critter-movement code, the whole Lilypad crashes as well. In the case these are separated, again only the critter crashes and is restarted.
