acoelomates
Acoelomates dont have a body cavity or an inner openning. Unlike, a coelom, has fluid-filled body cavity that is completely lined with mesoderm tissue. It serves as cushion for the internal organs of the organisms. It is basically found in bilateral animals and vertebrates.
Aside from serving as a storage unit for internal organs, the coelom also allows the internal organs to move and grow independently of the outer layer of the body wall.