What term describes animals that lack any sort of body cavity?

The term "Acoelomates" is used to describe animals that do not have a body cavity, which is a fluid-filled space between the digestive tract and the body wall. Instead, acoelomate animals, such as flatworms, have their organs and tissues embedded in a solid mass of mesodermal tissue. This lack of a coelomic cavity limits the complexity of organ systems and can impact the overall body plan and physiological functions of these organisms.

In contrast, coelomates have a true coelom, which allows for more complex body structures and organ development. Pseudocoelomates, on the other hand, have a body cavity that is only partially lined with mesoderm. Diploblastic refers to organisms that have two primary tissue layers, which is a different classification relating to developmental stages rather than the presence or absence of a body cavity. Hence, the defined characteristics of acoelomates specifically make them unique in lacking any kind of body cavity.