Eruvin 15: Live Animals May Wander Away

What if the lechi was there, in the right spot, with the necessary dimensions, naturally? What if that naturally occurring architecture element can be used as part of your sukkah? And, what if your naturally occurring architectural element is actually alive - not a tree, but an animal? The mishnah indicates that that would be fine, but R. Meir says otherwise. That said, writing a get (a bill of divorcement) on a live animal would be fine (R. Yosi HaGelili disagrees). Which leads to a list of other circumstances for which a live animal is not acceptable. Plus a shout-out to Ne'ot Kedumim. Also, a basic run-down of the details of how we arrive at the need for a written get. Plus the case of a caravan of people camping out who make their enclosure from the stuff they have with them. What if they can't quite finish the enclosure? A certain amount of a gap is acceptable, but no more. Bottom line: The length of wall must exceed the gap - because Hashem taught Moshe that detail (in contrast to "halakhah le-Moshe mi-Sinai," for example). What does this really mean?!