the most long-lived

The tortoise is the longest living animal in the land.

or maybe you mean…

The tortoise is the longest living terrestrial animal.

Why should we use "in the land." instead of "on the land" ?

On the land isn't a common phrase in English. What did you mean by it?

How about " on land" , "in water" ?

Either one could be correct in some situations, depending on what was said before the sentence. Alone, the second is much better. However, I think "longest-lived" would be much better than "more long-lived".

Ah, Aspara Gus is right--"on land" would be more correct. When comparing the longest-lived land and water animals, it is usually "on land" and "in the sea".

Or land-dwelling and sea-dwelling.

My answers all assumed that you meant the longest-lived animal that doesn't live in the sea. In that case "on land" is best, but "on the land" is also correct. Aspara Gus thought you you meant "in the whole world", because "in the land" is a phrase can be used to mean that. Aspara Gus might be able to explain that phrase better than me, though!

Jenny GrayAspara Gus might be able to explain that phrase better than me, though!

That's pretty much what I meant.

On land works, too. I used terrestrial (living on the ground) in the interest of expanding the OP's vocabulary.

but, you told me On the land isn't a common phrase in English.

It made me confused.