What does the term acoelomate refer to?

The term acoelomate refers specifically to animals that lack a coelom, which is a body cavity between the gut and the outer body wall. In acoelomates, the space between these two structures is filled with tissue, often mesenchyme, rather than a fluid-filled cavity. This anatomical characteristic can be seen in organisms such as flatworms (Phylum Platyhelminthes).

In contrast, the other terms describe different types of body plans. For instance, a body cavity lined by muscle tissue indicates a coelomate organism, while the presence of blood vessels alongside a body cavity lined by muscles suggests a more complex structure often found in other animal phyla. The notion of a full digestive tract does not pertain specifically to body cavities, as it can apply to both coelomates and acoelomates. Understanding these distinctions is crucial for grasping how different animal groups have evolved and adapted their body plans.