What term is used to describe an organism that lacks a body cavity?

The term used to describe an organism that lacks a body cavity is "acoelomate." In biological classification, acoelomates refer to animals that do not have a coelom, which is a fluid-filled body cavity lined by mesodermal tissue. These organisms are typically solid and have their internal organs embedded in a solid mass of tissue.

For example, flatworms are classic examples of acoelomates. Their anatomy showcases how their body structure allows for nutrient distribution and waste removal without the need for a cavity, as these functions are carried out via diffusion through their body surface.

In contrast, coelomates possess a true coelom, which provides a more complex and efficient structure for organ development and functioning. Pseudocoelomates have a body cavity that is not completely lined by mesoderm, and eukaryotes refer to organisms with complex cells that contain a nucleus but do not specifically relate to body cavity classification.