I guess the web search messed up then. You should search for biology stuff! Not computer science stuff! Turtles are very computer science related creatures, but there's more to them!

Turtle Mode is actually a complicated thing! It's often implemented in two ways: Cryptodira are the turtles that just pull their heads in, while Pleurodira kind of tuck their head inside sideways.

But it gets even more complicated with certain ancient turtle species. For example, we have the snapping turtles. Technically cryptodira, but they don't really enough shell to cover themselves. Not that they need to because they'll fucking go snappity snap on everything on range. Or we have things like the megacephalum which again is a cryptodira turtle but probably can't retract the head because did we mention it's called the Megacephalum.

EDIT: Forgot: snapping turtle copypasta