The evolution of which characteristic allowed for the development and enlargement of complex internal organs?

The evolution of a body cavity allowed for more complex internal organs to exist. The body cavity provides a space for the organs to be contained in and perform their functions. In all complex organisms, the body cavity that protects the organs is strong and spacious enough to prevent organs from squishing into one another, while being congested enough to prevent them from moving haphazardly every time the organism moves.