acoelomates
Acoelomates are animals that lack a coelom, which is a fluid-filled body cavity located between the digestive tract and the outer body wall. Instead of a coelom, acoelomates have a solid body structure filled with tissue. This group primarily includes organisms such as flatworms and jellyfish, which have simple body plans.
These animals typically have a bilateral symmetry and are often found in aquatic environments. Acoelomates rely on diffusion for the transport of nutrients and waste, as they do not have specialized circulatory systems. Their simple body structure allows them to adapt to various ecological niches.