The given sequence seems to follow a pattern based on how each number is described. Let's analyze it:

Starting with 1, the next number is described as "11" because there is one occurrence of the digit 1.
The next number, "21," is described as two occurrences of the digit 1.
The subsequent number, "1211," is described as one occurrence of the digit 2 and one occurrence of the digit 1.
Following that, "111221" is described as one occurrence of the digit 1, one occurrence of the digit 2, and two occurrences of the digit 1.
Finally, "312211" is described as three occurrences of the digit 1, two occurrences of the digit 2, and one occurrence of the digit 1.

From this analysis, we can see that each number is described in terms of the count of consecutive digits in the previous number. Therefore, the next number in the sequence would be described as:
"13112221" - one occurrence of the digit 3, one occurrence of the digit 1, two occurrences of the digit 2, and two occurrences of the digit 1.

So, the next number in the sequence is 13112221.