Final answer:
Flatworms in the phylum Acoela lack a true body cavity, or coelom, and rely on diffusion for gas exchange. They have an incomplete digestive system and distribute nutrients and gases through their gastrovascular cavity.
Step-by-step explanation:
Flatworms in the phylum Acoela lack a true body cavity, or coelom, making them acoelomates. This means that they do not have a fluid-filled space between their body wall and digestive system. Instead, they have a solid body structure.
The lack of a coelom affects the organization and function of their internal organs. For example, flatworms rely on diffusion for gas exchange instead of having a specialized respiratory system. They also have an incomplete digestive system, with only one opening for both ingestion and excretion.
In addition, flatworms lack circulatory and respiratory systems. Instead, they distribute nutrients and gases through their gastrovascular cavity, which is a network of branching tubes that extends throughout the body.