Ah yes, that would make sense. I can't recall exactly which I made, but I definitely used mostly worm parts. I assumed that it going exclusively in the one direction was a glitch. I think the missing link for my understanding was knowing that I needed to make a creature with a unique combination of parts to accomplish the task at hand, but didn't know what that was supposed to look like for one part (such as worm head, arm, etc) let alone how those parts would work together in a specific combination.
Sounds like you're on the right track though! Maybe even doing something like unlocking the ability to synthesize more parts comes later, and the player is forced to only synthesis the legs and main body to get the basics down first.