Acoelomate is the absence of a fluid-filled body cavity between the body wall and the digestive tract. This can have some catastrophic consequences. While fluid compression is minimal, the tissue around these organs of animals will compress.
Flatworms and other acoelomate creatures have no body cavity. Between the stomach and the body wall, semi-solid mesodermal tissues retain their organs in place.
As a result, acoelomate organs are unsafe to compress forces applied to the animal’s exterior. The coelom uses to diffuse gases, metabolites, and other substances. Due to dorsoventral flatness, the surface area to volume ratio of these organisms is big enough to allow absorption of nutrients and gas exchange by diffusion only.
Coelomate animals or Coelomata have a coelom with a whole peritoneum covering formed from mesoderm.
Organs can be linked to each other and placed in a certain order while still moving freely within the cavity thanks to the entire mesoderm lining. Coelomates are bilateral animals that include all vertebrates.