Identify the cavity that develops entirely from the mesoderm.

I believe it would be mesentery

Coelom.

Explanation:

The coelom can be defined as a main body cavity, which is developed entirely fro the mesoderm layer. It is present in nearly all animals that contains and surrounds the organs of digestive tract.

During embryonic development, coelom can develop from mesoderm by two ways. In protostomes, coelom is formed by splitting of mesoderm cells, while in deuterostomes, coelom is formed due to combining of mesoderm cells.

Thus, the correct answer is 'coelom.'